Who needs a number, and when
The number is never one number with one deadline. The caterer needs heads and meal splits days before the party to buy in the right shape. The venue needs a count to set tables, and seating needs to know who sits with whom. You need it on the morning, for the toast, for the favours, for whether the hall feels full.
A total dragged out of a chat scroll serves none of them, because it has no way to be current. What a number with deadlines needs is something that counts on its own, continuously, against names, with the party size and the plate already resolved.