We staff catering jobs according to:
1) Number of people to be served, and, to a lesser degree,
2) Complexity of menu
For a typical event (say 150 ppl, 2 meats, 2 hot sides, and 2 cold sides) my wife and I will bring 2 people along. The extras will start at about 3 PM with setup of the buffet, then misc prep like slicing buns, filling condiment bowls, and helping process the food as it comes off the pit (chicken into pans, pulling pork, etc). They help with cleanup, so they're usually on the job for about 5 hours.
Bigger events means more staff required (for example, 2 buffet tables = more runners and people needed in the kitchen trailer during service time). If we're unsure (big crowd, or lots of menu items, etc) we always error on the side of overstaffing. An extra $60-90 in labour costs is a lot better than spoiling an event because the food's late or we can't keep the buffet stocked. They're paying not just for the food but for the service as well.
