A quick rule for parties, weddings and jobsites.
100 guests for 4 hours = 400 guest-hours = 2 units.
A 5-unit party with a bar needs 6. Add 1 more for a full meal.
At least 1 accessible unit per cluster. 1 handwashing station per 4 toilets.
Cal/OSHA §1526 sets the minimum for California construction crews.