This depends on the atom in consideration. If we take the most common element on Earth, oxygen, for example, the atomic number of oxygen is 8, which means that an oxygen atom has 8 protons in its nucleus. The number of electrons in an atom is equal to the number of protons, so an oxygen atom has 8 electrons.

Of these 8 electrons, 2 are in the first energy level, closest to the nucleus, 4 are in the second energy level, and the remaining 2 electrons are in the third energy level, which is the outermost energy level.

Therefore, in an oxygen atom, there are 2 electrons outside the nucleus.