Calculate the number of covalent bonds in $H_2O$,$NH_3$,and $CCl_4$. Why?

(N/A) The number of covalent bonds in a molecule is determined by the number of shared electron pairs between atoms to achieve a stable octet or duplet configuration.
$1$. In $H_2O$: The central oxygen atom shares two electrons with two hydrogen atoms,forming $2$ $O-H$ covalent bonds.
$2$. In $NH_3$: The central nitrogen atom shares three electrons with three hydrogen atoms,forming $3$ $N-H$ covalent bonds.
$3$. In $CCl_4$: The central carbon atom shares four electrons with four chlorine atoms,forming $4$ $C-Cl$ covalent bonds.
The number of bonds is determined by the valency of the central atom and the requirement to complete the valence shell of each atom involved in the bonding.