looks like 6 to me. -copypasting following.
Electrons per shell 2, 8, 18, 2
http://en.wikipedia.org/wiki/File:Electron_shell_030_Zinc.svg
http://en.wikipedia.org/wiki/Electron_shell
Subshells
Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. For example, the first (K) shell has one subshell, called "1s"; the second (L) shell has two subshells, called "2s" and "2p"; the third shell has "3s", "3p", and "3d"; and so on.[1] The various possible subshells are shown in the following table:
Subshell label ℓ Max electrons Shells containing it Historical name
s 0 2 Every shell sharp
p 1 6 2nd shell and higher principal
d 2 10 3rd shell and higher diffuse
f 3 14 4th shell and higher fundamental
g 4 18 5th shell and higher
Number of electrons in each shell
s p d f g Total
K 2 2
L 2 6 8
M 2 6 10 18
N 2 6 10 14 32
O 2 6 10 14 18 50
An atom's electron shells are filled according to the following theoretical constraints:
Each s subshell holds at most 2 electrons
Each p subshell holds at most 6 electrons
Each d subshell holds at most 10 electrons
Each f subshell holds at most 14 electrons
Each g subshell holds at most 18 electrons
Therefore, the K shell, which contains only an s subshell, can hold up to 2 electrons; the L shell, which contains an s and a p, can hold up to 2+6=8 electrons; and so forth. The general formula is that the nth shell can in principle hold up to 2n2 electrons.