How many electrons are on the valence shell of an element with 12 electrons?
The valence shell is the last shell which the element's electrons take up. (In this case Magnesium)
In the first shell you can fit 2 electrons, 8 in the second, 8 in the third
10 electrons would take up the first two and two would go in the third. As the third shell is the last shell which have electrons therefore there are 2 valence electrons