Do you mean ACP -> User Search, then fill in a range of registration dates? That will certainly be close, but it's not the number of registered users, it's the number of user who registered. It would be slightly larger number because some user accounts have been deleted, other banned, etc.
Closer to what I'm looking for is user_state = Valid, Not Banned, and as of a 2020-12-28. I don't see way to do this from the user search screen. Apologies if I'm overlooking something.
It's easy enough to change the end date to 2020-12-28.
What I've done so far is: User State = Valid, Status = Not Banned, and an end date as shown above. When I allow all user states and statuses, my total goes from 14k to 18k users.
I still don't think, however, I've accurately accounted for things like users who asked their accounts be deleted. Maybe I have ...