There isn't a federal number for wedding guests, so a 9 to 10 count should come from the rental operator's own usage chart, not from a rule. OSHA 1926.51 only covers workers, and the 2010 ADA doesn't set a per-guest count either. What the ADA does set is in section 213.2: where portable units are clustered at one spot, at least 5 percent of them, and no fewer than one, must be accessible. So one ADA unit in the bank is the floor, and if you split units into two spots, each cluster needs its own.
When we did my dad's celebration of life out at the farm, the things that actually mattered were simpler than the count. Give the operator your guest number, the hours, and the open bar, and have them size it from their chart in writing. Put the units close enough to the tent that people aren't walking through dark grass, and get lights on them if the party runs after sunset. Ask for a handwash station next to the bank so people can wash with real water. Leave a clear lane for the truck, and ask whether they want to check or pump during the evening with eight hours of bar traffic.
If comfort is the goal, a restroom trailer needs power and level ground, so confirm both before you pick it over standard units.
