The number of protons in Aluminium is 13. The 27 means the atomic mass is 27 . The number of protons plus the number of neutrons is 27. That means the number of neutrons is 27–13=14.
Number of neutrons depends on the isotopic form of aluminium. More than 99.9% of naturally occurring aluminium is 13Al27.
Mass number, A = no. of protons + no. of neutrons. 27 – 13 = 14.