I am running an event that has a locker where participants can charge their phones. The locker has about 30 spots for phone chargers and was wondering what kind of generator I would be needing to make sure I had enough power to charge all 30 phones at once.

Pretty small generator. Even if the chargers draw 5-7 watts which is a lot, you would only need 150-210 watts. Most small generators put out 800-900 watts at 120VAC, 7.5 amps or so..