It actually depends on the size of the glasses! And what else is being served.

But basically, you need 1/2 cup icecream and 10 ounces of rootbeer for a nice basic float. Don't forget the spoon and straw.

If it is adults or families, you will want 1 each. If it is teens, allow about 1.3 each, since people will want seconds.

You know to fill the glass 3/4 with soda, then the ice cream then fill the rest of the way, to avoid too much overflow.