The atomic number of the atom = 15
Number of protons = 15
A number of electrons = 15 because the atom is neutral.
So representing the electrons by electronic configuration,
K = 2 electrons
L = 8 electrons
M = 5 electrons
The M shell has 5 electrons and it is the last shell of the atom, so the number of the electrons in the last shell of an atom with atomic number 15 is 5.