You aren't going to find rules on that in the rulebook. They're going to be in your army codex. If the BT stands for Black Templars, and you're working from the Black Templars codex, I'm not too sure about how many special weapons you get. But rest assured that there is a cap of one or two special weapons (don't go gluing special weapons on everywhere, you'll get two, maybe three).
If you're going off of the normal Space Marine codex, then a basic tactical squad can have one (1) special close combat weapon, and that's for the sergeant. If the squad is at full strength (10 men), then you can also have one (1) special weapon (plasma, melta, flamer) and one heavy weapon (missile launcher, heavy bolter, etc.).
Basically, you should probably pick up your army codex before you start building stuff, so you know what options are and aren't allowed.
