My OSM selection script:

area[admin_level=2][name="Česko"];
way[highway][junction=roundabout](area) -> .all;
make counter num = 0 -> .count;
foreach.all {
  if (! lrs_in(u(id()), visited_ids.set(t['ids'])))
  {
    complete {
      node(w);
      way.all(bn);
    }
    make visited
      ids = lrs_union(set(id()), visited_ids.set(t['ids']))
      -> .visited_ids;
    .count convert counter num = t['num'] + 1 -> .count;
    make stat circ=sum(length()),wids=set(id());
    out;
  }
}
.count out;

Warning: Not all of these are actual roundabouts, unless I manually checked them (see icon)! The links point to one of consecutive line segments that have the junction=roundabout property, and the number is their total length in meters!

Part 2 ► | Part 3 ►►

1243,323 m //TOP 00 Ostrava-Přívoz [D1]×[I/56]
1236,993 m //no C01 sign Úsilné [D3]×[I/34]
1199,566 m //TOP 01 Ostrava-Svinov [D1]×[I/11]
1110,363 m //[highway=construction]
937,861 m //[highway=construction]
859,544 m //[highway=construction] dammit, why are we even building new highways???
730,454 m //TOP 02 Ostrava hl.n. [I/56]×[II/647]; bonus points for an unofficial cul-de-sac roundabout inside
717,208 m //TOP 03 Hradec Králové-Plotiště n/L [D11]×[I/35]×[I/11]
656,361 m //TOP 04 Opatovice n/L [D35]×[I/37]; largest you’ll find described as “largest” in media
632,573 m //MANUAL: area[name="Náměšť na Hané"]; way[name="Hrad"](area) -> .all; zámek Náměšť na Hané 🏰
578,223 m //[highway=proposed]if that’s what it takes, I’m proposing to encircle the entire country in high speed rail
562,525 m //TOP 06 Opava-Malé Hoštice [I/11]×[I/55]
508,368 m //TOP 07 Brno, Slovanské nám.; town rectangle with park
487,721 m //TOP 08 České Budějovice, nám. Přemysla Otakara II.; literal, barren town square
450,956 m //TOP 09 Ostrava-Mariánské Hory [II/470]×[II/647]
445,589 m //TOP 10 Plzeň-Jižní předměstí [I/27]×Folmavská
435,249 m //TOP 11 Plzeň-Nová Hospoda [I/26]×Folmavská
424,798 m ) //TOP 12 Časy [D35]×[I/36]
406,55 m //MANUAL: area[name="Havířov"]; way[junction=circular](area) -> .all; Rondel Havířov [I/11]×[II/475]
401,29 m //TOP 14 České Budějovice-Světlík [I/34]×[II/634]
393,052 m //[highway=construction]
369,076 m //[highway=proposed]
352,274 m //TOP 15 Frýdek-Místek-Bahno [D48]×[I/56]
350,704 m //TOP 16 Uničov, Masarykovo nám., town trapezoid
349,701 m //TOP 17 Rumburk-Pražská [I/9]×[II/263]
340,176 m 2 regular ones, OSM error
330,095 m //TOP 18 Modřice-Decathlon [D3]×[II/152]
321,686 m //TOP 19 Praha-Chodov, Litochlebské nám.
320,525 m //TOP 20 Praha-Chodov, Horní Roztyly-Westfield
319,749 m
318,428 m
316,409 m
314,548 m
310,452 m
302,83 m
302,676 m
301,835 m
301,502 m
299,473 m
290,541 m Praha-Dejvice, Vítězné náměstí (Kulaťák) - Google thinks this is the biggest one, lol
288,75 m
288,493 m
288,248 m
287,169 m //[highway=construction] traffic peanut! 🥜 First in country? Jivno [I/34]×[II/634]
285,816 m
285,358 m
280,815 m
277,924 m
274,537 m
267,542 m
265,443 m
256,774 m
254,253 m
253,741 m
253,49 m
252,515 m
252,452 m
249,304 m
248,891 m
245,692 m
238,742 m
236,662 m
234,46 m
233,957 m
233,882 m
232,679 m
228,321 m
222,969 m
222,217 m
221,342 m
220,291 m
219,886 m
218,16 m
218,086 m
217,452 m
211,977 m
210,808 m
209,791 m
209,54 m
207,088 m
206,672 m
206,042 m
205,371 m
202,178 m
201,382 m
200,672 m
200,051 m
199,431 m
199,012 m
196,548 m
196,431 m
196,232 m
193,53 m
193,339 m
192,183 m
192,046 m
191,663 m
191,381 m
190,061 m
188,097 m
187,915 m
187,678 m
186,76 m
184,929 m
182,863 m
182,481 m
179,572 m
177,071 m
176,836 m
176,829 m
176,691 m
176,429 m
174,812 m
173,465 m
172,491 m
172,311 m
172,142 m
170,633 m
168,694 m
168,603 m
168,568 m
168,386 m
168,3 m
168,072 m
167,724 m
167,494 m
167,065 m
166,348 m
165,997 m
165,993 m
165,739 m
163,63 m
163,518 m
163,472 m
163,395 m
163,308 m
162,533 m
162,121 m
161,734 m
161,001 m
159,855 m
159,034 m
158,249 m
158,047 m
156,868 m
156,83 m
156,749 m
156,482 m
156,399 m
155,188 m
155,106 m
154,621 m
154,488 m
154,431 m
154,058 m
153,918 m
152,583 m
152,214 m
152,208 m
151,91 m
151,686 m
151,138 m
150,5 m
149,75 m
148,329 m
148,071 m
146,826 m
146,436 m
146,391 m
145,823 m
145,793 m
145,665 m
145,141 m
145,141 m
144,539 m
143,951 m
143,453 m
143,45 m
143,234 m
143,143 m
142,973 m
142,435 m
142,384 m
142,373 m
142,323 m
142,319 m
142,138 m
141,965 m
141,843 m
141,755 m
141,732 m
141,408 m
140,944 m
140,943 m
140,894 m
140,502 m
140,287 m
140,233 m
140,175 m
140,046 m
140,044 m
140,013 m
139,954 m
139,916 m
139,825 m
139,824 m
139,448 m
139,288 m
139,16 m
139,012 m
138,867 m
138,766 m
138,471 m
138,47 m
138,461 m
138,224 m
138,185 m
138,15 m
138,078 m
138,013 m
137,992 m
137,577 m
137,489 m
137,294 m
137,252 m
137,186 m
137,071 m
136,997 m
136,97 m
136,819 m
136,688 m
136,553 m
136,125 m
135,93 m
135,675 m
135,355 m
135,124 m
134,797 m
134,477 m
134,105 m
134,008 m
133,979 m
133,808 m
133,638 m
133,529 m
133,424 m
133,308 m
133,248 m
133,171 m
133,022 m
133,02 m
132,829 m
132,671 m
132,373 m
131,446 m
131,089 m
130,939 m
130,908 m
130,847 m
130,844 m
130,838 m
130,766 m
130,489 m
130,338 m
130,257 m
130,239 m
130,127 m
129,778 m
129,538 m
129,232 m
129,198 m
129,052 m
128,966 m
128,569 m
128,45 m
128,369 m
128,368 m
128,238 m
127,69 m
127,595 m
127,563 m
127,521 m
127,515 m
127,505 m
127,148 m
127,111 m
126,651 m
126,555 m
126,418 m
126,198 m
126,127 m
125,918 m
125,742 m
125,738 m
125,449 m
125,424 m
125,366 m
125,278 m
124,898 m
124,645 m
124,245 m
124,15 m
124,126 m
124,072 m
124,031 m
123,78 m
123,642 m
123,639 m
123,617 m
123,615 m
123,577 m
123,48 m
123,457 m
123,443 m
123,384 m
122,913 m
122,739 m
122,471 m
122,468 m
122,452 m
122,135 m
122,08 m
121,834 m
121,744 m
121,676 m
121,592 m
121,557 m
121,494 m
120,924 m
120,744 m
120,645 m
120,612 m
120,598 m
120,402 m
120,21 m
120,184 m
120,123 m
120,105 m
120,022 m
119,883 m
119,803 m
119,799 m
119,77 m
119,757 m
119,7 m
119,458 m
119,356 m
119,242 m
119,235 m
119,093 m
118,816 m
118,715 m
118,571 m
118,433 m
118,287 m
117,832 m
117,744 m
117,557 m
117,45 m
117,128 m
116,779 m
116,724 m
116,37 m
116,181 m
116,076 m
116,015 m
115,952 m
115,556 m
115,498 m
115,258 m
115,223 m
115,165 m
115,157 m
115,154 m
115,035 m
114,775 m
114,717 m
114,41 m
114,186 m
113,678 m
113,514 m
113,459 m
113,458 m
113,443 m
113,386 m
113,154 m
113,131 m
113,062 m
112,932 m
112,92 m
112,865 m
112,655 m
112,547 m
112,25 m
112,194 m
112,179 m
112,062 m
111,921 m
111,9 m
111,874 m
111,732 m
111,696 m
111,69 m
111,636 m
111,539 m
111,47 m
111,389 m
111,386 m
111,227 m
111,214 m
111,136 m
111,102 m
110,958 m
110,831 m
110,671 m
110,666 m
110,615 m
110,367 m
110,36 m
110,343 m
110,336 m
110,237 m
110,095 m
110,014 m
109,998 m
109,991 m
109,982 m
109,979 m
109,935 m
109,927 m
109,749 m
109,716 m
109,687 m
109,662 m
109,651 m
109,645 m
109,569 m
109,52 m
109,493 m
109,431 m
109,4 m
109,398 m
109,377 m
109,339 m
109,335 m
109,244 m
109,12 m
109,079 m
109,061 m
109,028 m
108,842 m
108,811 m
108,719 m
108,71 m
108,651 m
108,635 m
108,573 m
108,543 m
108,502 m
108,451 m
108,433 m
108,425 m
108,413 m
108,336 m
108,288 m
108,278 m
108,215 m
108,201 m
108,123 m
108,045 m
108,038 m
108,019 m
107,997 m
107,995 m
107,96 m
107,958 m
107,951 m
107,946 m
107,94 m
107,857 m
107,778 m
107,767 m
107,761 m
107,602 m
107,53 m
107,5 m
107,318 m
107,266 m
107,265 m
107,234 m
107,168 m
107,114 m
107,068 m
107,032 m
107,03 m
106,895 m
106,865 m
106,863 m
106,861 m
106,856 m
106,807 m
106,792 m
106,773 m
106,687 m
106,674 m
106,67 m
106,661 m
106,633 m
106,606 m
106,567 m
106,499 m
106,432 m
106,345 m
106,31 m
106,299 m
106,29 m
106,243 m
106,136 m
106,128 m
106,126 m
106,047 m
106,043 m
106,003 m
106 m
105,959 m
105,923 m
105,902 m
105,819 m
105,677 m
105,669 m
105,657 m
105,64 m
105,604 m
105,485 m
105,479 m
105,444 m
105,308 m
105,229 m
105,139 m
105,134 m
105,128 m
105,064 m
104,948 m
104,894 m
104,89 m
104,83 m
104,79 m
104,78 m
104,772 m
104,744 m
104,709 m
104,694 m
104,607 m
104,591 m
104,494 m
104,441 m
104,34 m
104,289 m
104,271 m
104,175 m
104,172 m
104,159 m
104,142 m
104,142 m
104,141 m
104,139 m
104,122 m
104,041 m
104,01 m
103,949 m
103,942 m
103,916 m
103,874 m
103,79 m
103,781 m
103,618 m
103,563 m
103,562 m
103,495 m
103,445 m
103,234 m
103,165 m
103,106 m
103,029 m
103,002 m
102,881 m
102,809 m
102,752 m
102,735 m
102,726 m
102,677 m
102,603 m
102,551 m
102,526 m
102,501 m
102,492 m
102,455 m
102,382 m
102,354 m
102,323 m
102,322 m
102,222 m
102,121 m
102,07 m
101,999 m
101,978 m
101,944 m
101,913 m
101,799 m
101,77 m
101,754 m
101,748 m
101,564 m
101,548 m
101,528 m
101,478 m
101,38 m
101,348 m
101,311 m
101,268 m
101,256 m
101,25 m
101,212 m
101,206 m
101,166 m
101,101 m
101,096 m
101,048 m
101,025 m
100,98 m
100,859 m
100,803 m
100,757 m
100,751 m
100,725 m
100,711 m
100,633 m
100,621 m
100,603 m
100,594 m
100,558 m
100,522 m
100,518 m
100,514 m
100,471 m
100,425 m
100,42 m
100,393 m
100,378 m
100,375 m
100,373 m
100,366 m
100,295 m
100,261 m
100,233 m
100,217 m
100,185 m
100,178 m
100,126 m

Whether a crossroad in the Czech Republic is officially, actually, really, undoubtedly, a roundabout, is decided SOLELY based on the presence/absence of the C01 sign , not OSM data! I am not responsible for your accidents.