How many electrons are present in the outermost shell of aluminium atom?

3 electrons are present in the outermost shell of aluminium atom.

Atomic number of aluminium is 13.  Electrons will be distributed in K, L, and M shells in the following way:
K shell can accommodate a maximum of 2 electrons.
L shell can accommodate a maximum of 8 electrons and
M shell will accommodate 3 electrons.
So, the outermost shell will have 3 electrons.