A typical 60x32" bathtub holds 70 gallons. A standard ~22oz box of brownie mix, when mixed with water, makes a little over a quart of batter. There are, obviously, 4 quarts to a gallon. Archimedes tells us to account for displacement, so really you'll probably only want to fill it about halfway, so 35*4=140 boxes. A quick perusal of recipe sites indicates that a flour:cocoa ratio of 2:1 is about right for what you want, but you'll need low-gluten cake flour to make it so you're not picking flour out of each others' hair for days afterward.
I do not, but I do recall that a large can of cream from Walgreens (which is a 14oz can) will fill the same mixing bowl well over the top, so you could probably just halve the result (i.e., 70) and have a spectacular time. Of course, spending $15 or so on a plastic mattress protector sheet and 20 or 30 cans of cream is much, much more fun and allows much more room for any extracurricular activities you might engage in. Maybe it's different for smaller people, but at 6'1" and quite a few too many pounds, doing anything with a girl in the bathtub other than taking a shower is not easy or pleasant. Plus, since shaving cream is just aerosolized soap, it washes right out of sheets and pillowcases, and cleanup on the floor/walls/headboard/etc is just a wet rag.
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