The sum of coordination number of aluminium ion and the number of bridging hydrogen atoms in ${\left[Al{\left(B{H}_4\right)}_4\right]}^{-}$ are respectively.

(a) The total number of ligands attached to a central ion is called the coordination number of that ion. The coordination number of aluminium ion ${\left[Al{\left(B{H}_4\right)}_4\right]}^{-}$ is 8, because eight ligands are   attached to aluminium ion.       

 ${\left[Al{\left(B{H}_4\right)}_4\right]}^{-}$ contain the complex ion $B{H}_4^{-}$ which is formed by $s{p}^3-$ hybridization of the orbitals of boron. In every tetrahedral structure of $B{H}_4^{-}$ ion is attached to aluminium therefore eight bridging hydrogen atoms are attached to central atom.