Will resolve based on FedIndex: https://fedi.wrm.sr/software/mastodon and their "active accounts tab" (as of 2026-04-06 it shows 750,920 accounts)

If their definition of "active user" changes or the site becomes unavailable, I'll try to find another site gathering the same statistics in the same manner (i.e. querying the nodeinfo API endpoint, regular crawling, respecting robots.txt) and if I cannot find any, this will NA.