how aluminium has covalency of 6 however boron has 4
 

Dear student,

Covalency of element is its total no. of electrons involved in the covalent bonding or no. of unpaired electrons available for covalent bonding.

Electronic configuration of Al is $\left[Ne\right]3s^2,3p^1$.
There are vacant 3d orbitals available so it can exhibit a covalency up to 6.
example: ${\left[Al{F}_6\right]}^{3-}$.
Electronic configuration of B is $1s^{2}2s^{2}2p^1$.
Here are one s and 3p orbitals are available for bonding, so covalency of B is only restricted to 4 as boron does not contain d orbitals in its valence shell.
So Boron has a maximum covalency of 4.

Regards