(Pay attention to the subject you're choosing, this is supposed to be in the Chemistry section, not here in the Mathematics)
On the periodic table, the Magnesium Mg is located in period 3, group 2. So it got 12 electrons.
So to become the cation Mg2+, the Magnesium has lost 2 electrons during it's formation.
So 12-2=10.
So there's 10 electrons in the Cation magnesium Mg2+
Hope this Helps! :)