Yummmmmm, Susan, banana splits! What a cute idea for your reception. With each guest making their own split, I would suggest you have plenty of ingredients on hand. I would also suggest that you use smaller-sized bowls so the first guests are a bit limited on how full they can fill their dishes. The quantities I have suggested below would seem to be to be minimums.

For the ice cream – there are 4 quarts in a gallon and each quart contains 4 cups (or 2 pints). Therefore each gallon contains 16 cups. The quantity of ice cream you use will depend upon how much ice cream you have in each banana split. You could get sixteen 1-cup servings per gallon, or thirty-two ½ cup servings per gallon. I think I would plan on having 3-4 gallons on hand.

I am not sure what sauces you want, but here are a few thoughts.

10 -15 cups whipped cream
8-9 cups Chocolate Flavor Syrup
50 - 75 maraschino cherries
4-5 cups canned salted Spanish peanuts

I hope this helps.