Best hardware:
GTX 1060 - 270-320 h/s
GTX 1070 - 400-480 h/s
GTX 1080 - 500 - 560 h/s
GTX 1080Ti - 700 + h/s
Find which one is best price / hash. Those are rough estimates, but you can get more info around.
how many should be good? like 6 GPU or more than
Go for as many gpu as possible, thats if your really that interested in mining btg.
The more gpu you can afford and run, the higher reward payout, obviously though.
But do you really believe in btg that much? And not use that hashing power towards another coin?
Well goodluck! I will not ignore btg just yet, and will also be pointing some hashing power towards this new coin and see where it goes.