Fewer porta potties with a mid-event service, or more units with none: which is smarter?

More units with no mid-event service is usually smarter for a 12-hour charity run and fair with beer than fewer units that depend on one pump-out.

A mid-day service empties tanks and restocks paper and chemical, which keeps each stall usable later in the day. It does not add doors during the rush after the race finishes or when the beer lines peak. If a couple of standard units fill before the truck arrives, you still get a line even though a service was booked.

On grass with easy truck access, a vacuum truck visit is easy to schedule, but the truck still needs a clear path and a few minutes at each bank. That can block a cluster right when people are walking past.

For all-day events with alcohol I order for peak concurrent use first, then add mid-event service only if the day is long enough that tanks or supplies would otherwise go south. Skipping service on a well-counted fleet is fine for many fairs. Counting on service to stretch a thin count is what creates the unhappy guest problem you described. Keep the ADA unit in the mix either way.

Megan is right, but she left out that you must check the park's special event permit for rules on service visits during operating hours. Some parks prohibit vacuum truck movement when the public is present. For your 12-hour event with 800 people and alcohol, the guideline is more than one standard unit per 50 people, plus the ADA-compliant unit. A mid-event service cannot increase unit count during peak use. Plan for enough units to handle the post-race and evening beer crowd without service. If the permit allows a truck and you have extra units, a 2 pm service is good insurance against full holding tanks but is not a substitute for adequate capacity.