Use the atomic number (Z) of the element and determine the electron configuration.
Z for Na is 11 => 11 electrons.
=> electron configuration = 1s2 2s2 2p6 3s
So, there you can count 5 s electrons: 2 in 1s, 2 in 2s and 1 in 3s).
The questions asks only for the 3s electrons, so the answer is one 3s electron.
