How many electrons does the 2nd main energy level hold when it is full?

8 electrons

Each principal energy level can contain up to 2n2 electrons, where n is the number of the level. Thus, the first level can contain up to 2 electrons, 2 (12) = 2; the second up to 8 electrons, 2 (22) = 8; the third up to 18, 2 (32) = 18; and so on.