A Pound Of Lead Or A Pound Of Feathers?

$\text{Molecules of }\ce{SO2}=(6.0221409\times10^{23})\dfrac{10_g}{64_{g/\text{mole}}}\approx9.40625\times10^{22}$

$\text{Molecules of }\ce{O2}=(6.0221409\times10^{23})\dfrac{10_g}{32_{g/\text{mole}}}\approx18.8125\times10^{22}$

+1 to Mateus Gomes who made the complete calculation.

But the answer does not ask 'how many' molecules are more numerous. So a simple math'' comparison helps to determine the more numerous specy - and spare the pen, the paper, its tree, or the wattovore (*) calculator - and incidentally the research of molar masses .

Sulfur has a non-zero weight. So $SO_{2}$ is heavier than $O_{2}$ .

So for a same number of molecules, $SO_{2}$ will be heavier than $O_{2}$ (is it called "molar mass"? We do not yet need in this item question).

So if we want some quantity of $SO_{2}$ be the same mass as a quantity of $O_{2}$ , we need more molecule $O_{2}$ than molecule $SO_{2}$ to "make" the same mass.

I forgot to say the preliminary check everyone may have done (but i tell in the end because it is not the core of the subject) : $SO_{2}$ is one molecule, $O_{2}$ is one molecule, so we won't be trapped nor blocked by the number of molecules of each gas, like if it had been the case in another question the same kind, with mixes of gases (gas mixtures?).

(*) i meant "energy-consuming", you understood the portmanteau neologism was shorter and practical...