the max is x4 the original damage
one stage= x1.5
e.g if you use Hone Claws 6 times it will make you Attack stat x4 higher.
1st time: x1.5
3rd time: x2.5
4th time: x3
5th time: x3.5
6th time: x4
a move/ability that lowers a stat does the same thing but divides by the number instead of multiplying by it.
hope I helped. if not, comment and i'll rephrase the answer