It’s actually not even a real political party, there is no socialist or social-democratic party in the US. IIRC DSA had ~12,000 members when Trump was first elected, in the middle of the coronavirus pandemic (and after Bernie Sanders dropped out of the democratic primary) there were 70,000 - 80,000 members. Since Biden’s election, and the 06Jan21 attack there are almost 100,000 now.

Those numbers are still paltry compared to other political groups, but the DSA is trying to build from basically nothing completely grass-roots. They are trying to unify left-leaning groups including democratic socialists and social democrats.