What it will do is for each item that has Overwhelming Power, it will add its number to the stack. That is if you have two items that give 20 per stack, each stack is now 40 per stack.

Can Azerite traits stack?

Azerite trait stacking is a good thing, and almost all traits are stackable. And remember, you can check bloodmallet to see how much each stack of an Azerite trait is worth for your specific spec. You can also check trinkets on there!

Does Firemind stack?

This effect stacks up to 3 times.