I would like to know how many companies make SxS rifled slug gun? Also why is it that there only one company that makes SxS slug guns that company is Galzan,so can someone tell why there not more?

Guessing here but limited market and high cost of manufacture? Double rifles are expensive to build and regulate. I bet H&H would build you a 12 bore DR for 100k or so. I believe someone is still making paradox guns, which might be an option.
C.G.B.