Can I lose slots?
No, once received, ship slots are permanent, and cannot be sold or lost.
How many ship slots do I have left?
You can see how many slots are not in use at the moment on the "Purchase a ship slot" button.
Do all ships need a slot?
No, different from other Wargaming titles, Premium Ships in World of Warships Blitz do not need a ship slot. So there is no need to ever sell a premium ship to make room for a different ship.