on the bottle it just says 500o IU, I am pretty sure it will not say per any ml. If you mix 1 ml or 10 ml of water with it, it will still be 5000 IU's of hcg. If you were taking it in 1 shot it would not matter, but since you only want to take 500 IU, you have to figure out the math for both the concentration per ML and how many IU's on the slin pin it would be.
Mr Ranks math above is correct if you use the 2 mls of water that came with it.