How are you building it? Are you doing a positive foam plug and just fiber glassing over it or are you making a mold from your foam plug then getting your final from laying fiber glass in that?
If you're making a mold, I'd say 3-4 layers: you want it flexible enough to get your product out.
If you're doing your final, especially if you just layered it on top of your foam. You probably want to do about 7 layers. I did about 3 of matt, then layered in some weave and then finished it with another couple layers of mat.
Words of advice though: get slow acting catalyst. Fast acting stuff (like the stuff you usually get in fiberglass repair kits) is too fast for the area you're working with. You'll probably wind up with air bubbles or an uneven finish. You'll have to sand either way, but it helps when you don't have a giant bulge in one spot.
Another big tip: resin, acetone and gelcoat will eat your foam... like... make it not exist in seconds. Make sure you laminate your foam plug REALLY well before you get into the harsh chemicals or all that time you spend sanding your foam into the perfect shape will be a giant waste of time.
