The electron configuration of an element typically indicates the number of electrons in the energy levels and sublevels in the atom's ground state (state of lowest energy). In a neutral atom, the numbers of protons and electrons are equal. The atomic number is the number of protons, so in a neutral atom it is also the number of electrons.
Valence electrons are the the electrons in the highest energy level in the s and p sublevels and are involved in chemical reactions (may vary with transition elements). The core electrons are all of the rest of the electrons in all of the previous energy levels.