how may layers of fiberglass should i use when making a rear lip?
i know most kits are 9 layers but not sure about just a lip!?

it's really up to you. Just realize that the more layers you have, the stronger it is.

When they advertise nine layers, they must use really thin, really light layers. Nine layers of 1.3 oz. mat or of conventional cloth(like you buy at most stores...not sure what weight it is) would be stronger than most sub enclosures, and would be very thick and heavy if you built a whole lip with that much thickness.

Do you plan to use cloth or mat for your project?

i am using both,
so far i started with cloth for the first layer
then i layed a layer of mat
then i layed a layer of cloth today
next probably another layer of mat then cloth tomorrow which
would be a total of 5 layers which seams good to me since i will have bondo w/fiberglass strands coating the outside to smooth it over and take out any dips and such

I wish I knew how to lay that stuff...I'd have a custom sub enclosure by now!!

i say keep adding layers untill you think its strong enough. and when you think its strong enough add some more layers