How Many Appetizers Do I Need for 125 People?

If you are buying them premade, ask the place you're dealing with or buying them from. They'll know exactly how many you'll need.

If however you are planning to make them yourself, depending on size, you'll need at least
4-6 appetizers (pieces) per person.
It it's a sit-down meal, then, of course, you could easily get by with 3-4 each person.
Two joints of a chicken wing or 2 bacon-wrapped shrimp, 2 fried sticks of mozzarella cheese.
Kinda depends on the type of appetizers too. Are they to be all cold? All hot? Chips and Dips? Crudites w/ Dip? You're not giving us enough information to give you a definite answer really.

Good Luck to you in all cases.

Pookarina