You mean how many ship models? Or how many “ships” ?
It’s very easy to modify your ship. All the systems are on cards and after each adventure you’ve hopefully made some money and can perhaps sell old kit or buy new kit, and thus you just add cards and now you have a new-ish ship. Eventually, though, you’ll reach a point at which your old ship is just too small for the cool expensive gadgets you want to add, and then it’s time to sell the old girl and buy something bigger and badder.