I think this needs performance testing to see if the increase has a negative effect on server hosting 24 slots. So 32 sounds reasonable for an initial release, though maybe it could be raised to 48 without too much loss of performance. (Nobody has said 48 but I just mention it to suggest a new value.)
I think though that by the time an update is due, we’ll have stronger servers able to handle 64 players. (Without crashing, that is. Don’t tell me it won’t lag.) So when updating, this value could as well be increased to 64 if servers don’t go belly-up when hosting only 24 slots.