Keeping your feet firmly planted in place while benching helps stabalize you during the lift. Also tucking your feet to an extent (used in conjunction w/ an arch typically, ie powerlifter style) can give you leg drive, helping your bench as well.
It's funny to watch what people start doing with their legs when they begin to struggle on bench. Often you see their knees wagging back and forth and their feet moving all over the damn place like they're dancing or some shit. This will do nothing but hurt your performance on bench.
As far as volume, it depends on multiple things, such as intensity, etc. Generally speaking I think 9-12 sets of chest is fine. Of course, some people respond better to low volume and vice versa. You'll just have to experiment some.
