Well, Nicole, you don't know how many you plan to serve, and you don't say what other refreshments are available, so I am kind of up he creek on trying to estimate.
Here are some things I can tell you.
Get enough ice and ice chests that the root beer is really cold before mixing.
Use a 12-14 ounce plastic glass with a straw and a plastic ice tea spoon for each one, and napkins available. Have an 8 ounce glass for kids under 12.
In the 14 ounce glass, add 8 ounce root beer, then a 1/2 cup scoop of ice cream, then root beer to top off, about 2 ounces. In the kid glass, 4 ounces soda, 1/3 cup scoop, top off with about 2 ounces.
1 gallon ice cream makes about 30 floats. A 2 liter bottle makes about 6 floats.
You need dry ice storage to hold the ice cream. DANGEROUS FOR CARELESS FOLK!!! Must be handled with gloves.
Have some sugar free root beer and some ice for folks who don't want floats. Adding iced tea and sweet ice tea will increase your profit at a very low cost.
Charge about 3-4 times what a float costs, or comparable to your local Sonic/ drive in. harge 1/2 that for the kids size.