What is 500ml in mg?

That depends what substance is in the 500 ml volume, and how much mass is in each ml of that substance.

If the 500 ml are full of stones or lead, they contain many many mg of mass.

If the 500 ml are full of water, they contain roughly 5,000 mg of mass.

If the 500 ml are full of air, they contain far fewer mg of mass.

If the 500 ml are empty, they contain no mg of mass at all.