Mathematical relation between number of GPUs and H/s

This is more of a theoretical question.

I need to know what is the mathematical relation between the number of GPUs being used and the cracking speed.

For example, for a fixed kind of GPU, I have X H/s. If I used 10 GPUs of the same kind, would I get 10X H/s?
If not, what would my speed be?

Is there any kind of rule or mathematical model that would give me that relation?