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