Guaranteed Guest Count
The final headcount you commit to pay for, given to the caterer or venue by a deadline.
A guaranteed guest count (the "guarantee") is the number of guests you confirm to your caterer or venue by a set deadline, usually a few days out. You pay for at least that many even if fewer attend.
Caterers staff and buy food against this number, so it cannot drop after the deadline (though you can often add). Track your RSVPs closely and set the guarantee with your caterer once replies are in.