The number of hydrogen atom(s) attached to phosphorus atom in hypophosphorous acid is:

Hypophosphorous acid has the formula ${\text{H}}_3{\text{PO}}_2$.Its structure consists of:$1$ $\text{P}=\text{O}$ bond$1$ $\text{P}-\text{OH}$ bond (mono-protic acidity)$2$ $\text{P}-\text{H}$ bondsThese two hydrogen atoms are directly attached to the Phosphorus atom, which gives the acid its reducing property.