The exact number is 2097152 GH = 2 PH.
There is no exact number, just estimates.
I am saying that 2 PH equals to exactly 2097152 GH, for those interested to capture exact moment.
It does not.
2 PH = 2000 TH = 2000000 GH = 2000000000 MH = etc... You get the idea.
Lets not propagate the sillyness of the computer-storage world where standard metric quantities are mutilated to the nearest power of 2. It's all neat powers of 10 in the SI unit system, like it's supposed to be.
Computer Science deals often with powers of 2. Same a s bitcoin.
Let's see that sillyness below:
If you use power of 2 then:
4GH/s produces 1 shares per second.
1TH/s produces 1 share of difficulty 8 per second.
In base of 10 you have:
4HG/s = 4,000,000,000 H/s would produces 0.931322574615478515625 shares per second
1TH/s would produce 0.9094947017729282379150390625 shares of diffuculty per second.
etc.
You get the idea.