I'm not exactly sure if you mean HS (hull spaces, which depends entirely on how big the ship is) or armor layers, which I think would be more relevant. This is one of things that will be different for everyone. I prefer to put as much mass as possible into offensive systems, which usually means that my most heavily armored ships have about 8-10 armor layers, while the median is probably closer to 4-6 layers. This will also change drastically with your technology, since armor takes up less space further up the tech tree. At starting technology, 3 or 4 armor layers is already significantly cutting into your mass budget, while at high levels, 15 layers might be small enough for every warship to use. It might also be useful to compare it to weaponry, especially missiles: A strength-4 warhead will pierce through 2 layers of armor, a strength-9 warhead will pierce 3, a strength-16 warhead will pierce 4, and so on. If you want a ship to just be able to take a couple of hits and keep fighting, that could give you a rough minimum based on what you are facing.