Out of 500mg magnesium oxide how much will be absorbed in MG?

Many facets of mag. most absorbable magnesium salts are mag.citrate,glycinate,taurate or aspartate.

60% of mag.oxide only contains elemental mag. Out of that only 23% is absorbed.

Avoid mag.carbonate,sulphate,oxide & gluconate due to poor absorption.
Wrong question. If you are given this by a qualified MD for valid medical indication the Dr will prescribe the type and dose of Mg. If you are taking it for any other reason you are a fool. If a Dr advised you to take it, but the Dr failed to specify the form and dose, you need a better DR.