2.5MG equal how many milliliter?

Answer 1

There is no sensible answer to this question. A millilitre is a measure of volume, with dimensions [L3]. A milligram is a measure of mass, with dimensions [M]. Basic dimensional analysis teaches that you cannot convert between measures with different dimensions without additional information.

For example, 2.5mg of a dense substance, like lead, will occupy far less volume than 2.5mg of air.

Answer 2

This cannot be sensibly answered. A milliliter (mL or ml) is a measure of volume, mg is a measure of weight or mass.