Start with your guest count and event length. A daytime gathering with short stays has different restroom demand than a wedding, festival, or concert where guests remain for several hours.
Food and beverage service can increase restroom use, while alcohol service may make additional capacity a sensible planning consideration. Keep ADA access and hand-washing near the main guest flow rather than treating them as afterthoughts.
Share the venue address, delivery path, event hours, and setup window with your rental company. Those details make the recommendation more useful than a one-size-fits-all number.