Venom is basically correct, there isn't too much of a performance hit if you use more than one map. There is a big benefit of using fewer maps though, you get a more uniform texturing scheme and it's easier to edit large areas of the model all at once.
The Morrigan started out with two maps:
This is the hull, gunpod and cockpit.
This is the engines and wings.
You'll notice I outlined the cockpit in the top image, I was unhappy with how the cockpit turned out so I redesigned it. I gave it a map of its own:
You'll notice that the texture map for the wings and engines have only one wing and one engine drawn. I overlapped both wings and both engines so what I draw for one automatically applys to the other, this saves a lot of time and makes editing the overall texture scheme for the fighter a whole lot more convenient. The real bonus is that I kept the original mapping scheme for the ship:
This is the template for the gun (the other parts are simply superimposed later). This can be quickly imported into photoshop or PSP and an entirely new texture for the gun can be built, this obviously applys to the rest of the model too so anyone can make a new texture map for the ship with ease.
Cool eh?
