In a buffer solution consisting $Na{H}_{2}P{O}_4$ and $N{a}_{2}HP{O}_4$

The more acidic is considered as acid. In this case $Na{H}_{2}P{O}_4$ is more acidic so it is acid. The other species is salt. The equilibrium constant to be used in hendersons equation for calculating pH should be one relating to the acid specices to its salt i.e., $H_{2}P{O}_4^{-}\stackrel{}{\rightleftharpoons }HP{O}_4^{2-}+H^{+}$  So $K_2$ is the relevant equilibrium constant and the given equation can be used to calculate pH .