This is a personal choice, of course, but there is such a thing as a too sloppy sloppy joe. I like to put a generous enough amount on the buns so I can really taste it and so maybe some will fall out (because they are sloppy, after all), but not so much that all of it falls out or that the bun disintegrates. My rule of thumb is to allow about 1/2 cup (or a heaping 1/3 cup) of sloppy joe mix per person.