Some Misskey instances do not report their MAU numbers. Misskey total MAU is undercounted because of this, but, looking at Misskey instance stats I suppose, that it's 20 000 at most.
All Misskey forks (Firefish, Iceshrimp, Cherrypick, Foundkey, Meisskey, Sharkey), that are listed by FediDB make another 10 000 MAU.