The number of units on an insulin pin to inject 500IU is dependent on the the the amount of hcg you reconstitut and the amount of solvent used.
I advise you to read the thread attached below and it you still have questions then create a new thread.
Also, it's not a good idea to use 5ml of bac water to reconstitut 5000IU of hcg. Imo 2-3ml is sufficient.
Mixing HCG. Why use a different vial?
Your algebra is incorrect.
If OP used 5ml of bac water to reconstitut 5000IU of hcg then 500IU would be 50 units on a slin pin.