most likely it would be ported of course. 15.6 sealed to me is underheard of. You can fit 2 to 4. I have 2 15's in a 16.56cuft box. Minimum cuft for my subs are 3.5cuft. Minimum for a lot of subs is probaly 3.5cuft-4cuft depending on sub.
It also depends on how the trunk is..... can you fit the box in there after building it, do you have to build it in there, can you still fit the subs in after building it in the trunk, ect.... but i would agree with berny, 2-4 tops after all displacement... .but you will have lots of fun trying to fit 4 in there for sure... 3 would be about tops with recc. space and displacments..4 would probally be pushing it
I'm sure he has a suv if he's going to build a box that big. And he can probaly make the box taller if need be. If designed right, you can do 4. I wouldn't do subs in odd numbers unless it was powered by its own amplifier.
I am planning on getting 2-4 Fi Q or Bl serries 15" subs, I dont know what series yet lol, I want amazing sq and great spl, but yeah, thank you for the replies