The maximum covalency of aluminium is ‘6’ where as that of boron is ‘4’ because

Al contains empty d-orbitals in its valency shell that is in  third shell,therefore it can extends its valency up to 6.Where as Boron doesnot contain empty d-orbitals in its valency shell and hence it cannot extends its valency to six and its maximum valency is 4,due to presence of 4 valency orbitals i.e.,one ns and three -np orbitals.