Generally speaking, it's hard to tell how many members a club exactly have, cause it's hard to quantify the "Karteileichen"- a German expression for "members", who haven't paid their fee for a while and still are counted, although they might not be interested anymore or even dead.
Transfermarkt is usually your best bet for up-to-date stats.
1. Bundesliga: https://www.transfermarkt.de/1-bundesliga/daten/wettbewerb/L1/sort/mitglieder.desc
2. Bundesliga: https://www.transfermarkt.de/2-bundesliga/daten/wettbewerb/L2/sort/mitglieder.desc
3. Liga: https://www.transfermarkt.de/3-liga/daten/wettbewerb/L3/sort/mitglieder.desc
Lol that last sentence just makes me think of Souness or something.
For what it’s worth, I can understand why they resigned him - even if consistency is an issue. With COVID, United’s revenue has been hit... like everyone’s has been... they’ve got other areas of the squad that will need more immediate investment.