How to type the hash key on an iPad pro?

The on-screen keyboard layout may differ due to your iPad model and language settings choices, but the method to access the # key (and others) is essentially the same.

Tap either ?123 key situated next to the space-bar:

I don't have an iPad but with an iPhone there's a pair of modifier keys at the bottom left that get you different keyboards.

Yes, as I said, a modifier key at the bottom left.