The reaction of  $C{l}_2$  gas with cold – dilute and hot concentrated NaOH in water give sodium salts of two different oxoacids of chlorine, “P” and “Q” respectively. The $C{l}_2$  gas reacts with $S{O}_2$  gas in presence of charcoal to give “R”. R reacts with white phosphorus to give a compound “S”. On  hydrolysis “S” give an oxoacid of phosphorus “T”.
The number of atoms in the molecule of “R” is ______