Since he is putting in 2 subs in, any reinforcement would be good since it's not MDF. Putting 7-8 layers will be pretty heavy. You can add any number of layers as long as you think it's strong enough, there isn't an accepted guideline.
Two 10W3's won't be too heavy compared to the v2, but 5-6 layers should suffice if you are planning to mold it in the back area. If you are using your spare tire cavity instead, you don't need that much since that area serves as a frame. You can even use some cloth fabric, lather it up with some resin and hardener, let it cure, and it will be pretty strong too. Just remember to add layers from the inside, not on the molded side, otherwise, it may not fit in there nicely.
well, i will be using the w3v2's and so far i've got 6 layers down and i put on some body filler and started to sand it. it starting to look pretty good. only thing is that i had to lay the f/g on the outside as i couldn't get into the inside, but were you talking about the box or the wheel well?
Anyway, once i finish with the sanding, i'll post up pics of the progress from beginning to end. thanks for all your help guys