Serving approx 75 to 100 people root beer floats in an 8 oz cup. How many 2 liter bottles of root beer would I need, and how many gallons of ice cream? About 5 gallons of ice cream, 1 2 liter per 10-12. I need to make this for 150 people can you give me the items needed and quantiy.ow many 2 liter bottles of root beer would I need, and how many gallons of ice cream? ellen 07/18/12 About 5 gallons of ice cream, 1 2 liter per 10-12. Olivia 09/27/12 I need to make this for 150 people can you give me the items needed and quantiy.