How many porta potties?

Start with 1 unit for every 50 guests.

Serving alcohol?

Add 1 unit for every 100 guests.

Party past 6 hours?

Add 1 more unit for every 100 guests.

Food or public event?

Add 1 hand-wash station per 4 toilets.

Example: 100 guests, beer, 5 hours

3 standard units and 1 hand-wash station.