Infinite how? Both countries have millions of military aged men, before either country actually runs out of manpower there would have to be a hundred Stalingrad battles. Clearly the population amount is not the bottleneck here. For either of them to use manpower as a “weapon” they’d have to throw naked men at machine guns hoping that bullets can’t be resupplied faster than they can draft.
Russia like any other nation has a very limited amount of manpower that they can leverage before consequences start to hit. So far they have avoided another mobilisation, but if the numbers dwindle enough they’ll have to do it and that is going to be very unpopular. They can’t afford to grind their men the way they’re doing but they’re doing it anyway, probably because they want to project the image of “endless manpower Russia” - if they can keep the farce long enough then maybe Ukraine’s allies will give up.
By believing this tankie nonsense you’re helping them. Don’t.