How much paint to buy?

Good evening,

I am going to be face painting with someone else (I have no prior contact with them) at a girls night out where 500 will be attending (as far as I last heard). Now, not all will be getting their face painted, but how does one figure out how much paint they will need for that many people? Is there a formula people use? I know types of paints are a factor. I use mostly paradise, but I may be trying some other brands out. Any help is appreciated.

Think that instead of trying to figure out how much paint would be needed for 500 you should figure out how long you will be painting at the event and how many faces you can do per hour spent there. Just my opinion.