Preparing for a messy session, and I need some help on how much to buy. How many boxes of cake mix (1lbs) do I need to fill up a 5 gallon bucket? And what's the ratio of water to cake mix?

around 18

I would go with ten but it does depend on how thick to want it.

5 boxes of cake mix and 1 box of pancake mix OR most of a normal size of flour. Then add luke warm water as needed as you mix.