With half, or less than half, of the population they have now, Russia mobilized approximately 35 million people for WWII.
They mobilized about 15 million for WWI. They didn't have anywhere near enough rifles for that many so they handed rifles from dead men to live men in the trenches.
That is why I think Ukraine better stop at the border when they get the Russians kicked out of their country. If they try to invade Russia for vengeance they will lose.