The equivalent mass of phosphoric acid $\left({\mathrm{H}}_3{\mathrm{PO}}_4\right)$  in reaction,

$\mathrm{NaOH}+{\mathrm{H}}_3{\mathrm{PO}}_4⟶{\mathrm{NaH}}_2{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}$ is

$2\mathrm{NaOH}+{\mathrm{H}}_3{\mathrm{PO}}_4 ⟶{ \mathrm{NaH}}_2{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O}$

Only one hydrogen atom of   is replaced, hence equivent 

mass of  ${\mathrm{H}}_3{\mathrm{PO}}_4$ is equal to  its molecular mass