Question

2Mg meant by what how many atoms and molecules​

Answer 1

Answer:

O2 means two oxygen atoms. The number before the element is the number of molecules. 2H2 means two molecules of hydrogen and four hydrogen atoms. 2Mg means 2 Magnesium molecules

Answer 2

milligrams cannot define the number of atoms/molecule in substances

every substance have different amount so you have to prefer which substance.