Backyard wedding 100 guests, 60% women: how many porta potties?

For 100 guests with an open bar and about 60 women over five hours, I would order four standard units plus one ADA unit, and I would not trust a skinny two-unit chart order.

The units per person guideline starts near one unit per 50 to 75 people for a four-hour event, then adds units when alcohol is served, when the reception runs longer, and when more of the crowd is women. Your setup hits all three, so peak demand after dinner and around speeches is where lines form if you under-order.

Swap one standard unit for a deluxe unit if you want a nicer stall for guests. A full restroom trailer is overkill for this size unless you already want that look and have power and leveling sorted.

On the ADA question: the ADA cluster rule in the 2010 ADA Standards says at least 5 percent of each cluster (minimum one) must be accessible at places of public accommodation. A private backyard wedding is not covered the same way, but once you cluster several units most planners still include one ADA unit so guests who need it are not stuck. Your level grass and truck access are fine for drop-off and pickup.