This all depends on the element's atomic number (number of protons). Since atoms have the same number of protons as they do electrons, the atomic number is essentially equal to the number of electrons as well.

For example, Aluminum atomic number of 13 in the first electron cloud it will only have 2 so then it will have 8 in the other layers until there are not enough electrons. So the first will have 2 electrons, the second layer will have 8 electrons and the last layer will have 3 electrons because there are not enough electrons to make another full layer of 8 electrons. Therefore, Aluminum has 3 valence electrons.