32 to be exact. 4*8=32, but nonetheless its extra space. However, I will have to say once again because they become dependent on this mobile fleet, if they suffer any losses, its going to really hurt them as compared to other races.
That's 8 imperial labs and 8 military labs, so 32x2, so yes, 64.
And while it makes the reliant on these capital ships to remain alive, they only need 4 to fully benefit from the ability, add in a couple extra just to make sure that when you lose one your tech trees stay unlocked.
However, in a singleplayer game, or playing with friends versus AI, I could see myself having a lot of fun scuttling my own planets and then just roaming the galaxy with my fleet.