NATO have shipped Ukraine ATGMs and MANPADs literally in the thousands. All we've seen so far of the captured ones have been anywhere between a handful to a couple dozen here and there.
Even if the Russians have captured hundreds of them, it would still be only about 1/10th of what Ukraine currently possesses.
That said, we also don't know how many have already been destroyed in warehouses and depots by Russian artillery and air strikes.
Russia has war economy. Capture a few, reverse engineer, mass produce. Iran reverse engineered and mass produced TOW.