If an atom has twelve electrons, how many electrons are in the n=1 energy level? how many electrons are in the n=2 energy level? how many electrons are in the n=3 energy level?

The shells correspond with the principal quantum numbers (n = 1, 2, 3, 4 ...) Each shell can contain only a fixed number of electrons. The first shell can hold up to two electrons, the second shell can hold up to eight electrons, the third shell can hold up to 18 and so on.