Factors to consider in your calculation:
1.- Determine the total time the restrooms should be available.
2.- Determine how many people will attend
4.- If there's a higher ratio of women-to-men-attending, add 15% to 20%
5.- Add one handicap restroom per 20 standard restrooms, or a minimum of one for events open to the public.
6.- Adequate hand-washing should also be provided with restrooms in all food service areas.