How many porta potties for a 10-hour outdoor work event with 120 people and lunch?

You need at least six standard units plus one ADA-compliant unit for this event. Carl is correct that OSHA 1926.51 gives a baseline, but a volunteer day with a catered lunch and no alcohol is closer to a general industry event under OSHA 1910.141. That standard requires six toilets for 120 employees. Since you're clustering the units, you must include an ADA-compliant unit to meet the ADA 5 percent cluster rule. A handwash station by the food service is sufficient; a deluxe unit with a sink is not necessary. With a ten-hour day, plan a mid-day service visit if the rental company offers it to prevent the units from becoming overwhelmed, especially after lunch.

For 120 adults on a 10-hour volunteer build with a catered lunch, start from several toilets rather than one or two, and put at least one ADA unit in the same cluster as the rest.

OSHA 1926.51 is aimed at construction crews, but the numbers still map cleanly onto a workday like yours. Once you are past 20 people on site, that standard calls for one toilet seat and one urinal per 40 workers. At about 120 people that is three seats and three urinals. A full day plus a meal will pile traffic on after lunch and again late afternoon, so a single standard unit will back up fast.

Keep the bank where the vacuum truck can reach it. Under the 2010 ADA Standards, when portable toilets are clustered in one spot, at least one unit in that cluster has to be an ADA-compliant unit (and roughly five percent of the cluster).

A handwash station by the food line is enough for most volunteer days. A deluxe unit with a built-in sink is nicer, but it is not required if washing is already next to lunch. Stage delivery the evening before or first thing in the morning so nothing is still being dropped when volunteers show up.