for BG only, usually 1-2... 3 max. some people like to layer their brushes for whatever reason. waste of time if you ask me.
depends lol i dont like to undo i like to del layer if not happy with it i can use up to 10 - 20 layers easy
exactly 1-2 for actual brushing, 1-2 for effects, 1 for render, and 1 for text, and 1 for border, and i just merge, i dont save psd's for my sigs. but for bigger stuff, i usually have like 20-35