The number of molecules of Oxygen in 1 litre of air (21% of oxygen by volume) at STP would be

100 mL volume of air contains 21 mL volume of oxygen

1000 mL (1 lit) of air $\to$? mL volume of oxygen

$\frac{1000\times 21}{100}=210\text{\hspace{0.17em}mL}$ volume of oxygen

$1\text{\hspace{0.17em}mole}=22,400{\text{\hspace{0.17em}mL\hspace{0.17em}of\hspace{0.17em}O}}_2\to 6.023\times {10}^{23}{\text{of\hspace{0.17em}O}}_2$

$210{\text{\hspace{0.17em}mL\hspace{0.17em}of\hspace{0.17em}O}}_2\to ?{\text{\hspace{0.17em}molecules\hspace{0.17em}of\hspace{0.17em}O}}_{\text{2}}$

$=\frac{210\times 6.023\times {10}^{23}}{22400}=5.6465\times {10}^{21}$