If I can remember correct, there are 8 valence electrons in the outter shell.
Below is a quote from Wikipidea:
The valence shell is the outermost shell of an atom in its uncombined state, which contains the electrons most likely to account for the nature of any reactions involving the atom and of the bonding interactions it has with other atoms. The outermost shell of an ion is not commonly termed valence shell. Electrons in the valence shell are referred to as valence electrons. The physical chemist Gilbert Lewis was responsible for much of the early development of the theory of the participation of valence shell electrons in chemical bonding. Linus Pauling later generalized and extended the theory while applying insights from quantum mechanics.
In a noble gas, an atom tends to have 8 electrons in its outer shell (except helium, which is only able to fill its shell with 2 electrons). This serves as the model for the octet rule which is mostly applicable to main group elements of the second and third periods. In terms of atomic orbitals, the electrons in the valence shell are distributed 2 in the single s orbital and 2 each in the three p orbitals.
For coordination complexes containing transition metals, the valence shell consists of electrons in these s and p orbitals, as well as up to 10 additional electrons, distributed as 2 into each of 5 d orbitals, to make a total of 18 electrons in a complete valence shell for such a compound. This is referred to as the eighteen electron rule.
Each shell (n=1, 2, 3, 4) can hold 2, 8, 18, or 32 electrons (2n2 electrons). It is important to note that the number of valence electrons is not necessarily equal to the total number of electrons in a given electron shell. For example, because the 3d subshell has a higher energy than the 4s subshell, the 3d electrons are considered to be part of the 4th valence shell. So, while the 3rd electron shell can contain a total of 18 electrons (2 in the 3s orbital, 6 in the 3p orbitals, and 10 in the 3d orbitals), the 3rd valence shell contains only 8 electrons as the 3d electrons are typically not part of the 3rd valence shell.