Time of year
Out of town guests
Are more of your guests local or out of town? And how far out of town are they? Will they have to fly or drive? Out of towners, especially those who have to purchase plane tickets, are less likely to travel, so that could decrease your overall guest count. If more guests are local, your attendance will likely be higher.
Tit for Tat
Did you attend their most recent simcha? Or did they attend another family member’s recent Bar or Bat Mitzah? When camp friends live far away, they are more likely to attend yours if you attended theirs. The same thing is true for family that is a bit more distant.
Families vs. Adults Only
There’s always a line that needs to be drawn with how many kids can attend. Do all adults who have kids get to bring the kids, even if their kids are significantly older or younger than the Mitzvah tween? If a parent is invited without their child and they have to get a babysitter, they are less likely to attend.
Last minute cancellations/No shows
Finally, you’ll always get a few people who have to cancel at the last minute. It’s super annoying, especially if you’ve already given your caterer the final guest count, but life happens.