As many as you want, but you can't do anything with the fourth.

if you add say an extra arm it wont work, well, it willbut the arm wont move, same for legs, feet and everything else

Not a lot, I misunderstood what you were saying.

"For example, Metal will stop a Heavy Digger, but not an AK. (So they can't dig into the brain room, but they can sure shoot through it.) Mega Metal isn't any better at that point."

I guess I took the complaint about the lack of armament on the brain room to mean that you desired a stronger material, and, Test is a significantly stronger one.

well if you're wondering about formulas you could go straight to data he could help. I like your idea of realism. Studying cortex command has sort of given me an inside look at how games are made. It very well be my full time job in the future.

Perhaps I will write a concise guide at some point and post it up... somewhere.