6 lat temu
You CAN, however, get a second CREW. In the case of a special crew, like the Zero-Skill BIA crews, you can buy the tank again, get the gold and garage slot, AND get the second BIA crew to retrain for any other tank.
I verified this a couple days ago when I bought the ISU-122S on day 13. I got the gold and garage slot, but since I already had the tank, I got the BIA crew in my barracks....
I will watch out for other tanks with special crews (like the Cromwell B) from other nations to stock up on better crewmen. Since I'm buying Gold anyhow, I may as well get a bonus for it.