russia has about 140 million people, but they're about 65% female. Not that women can't or won't fight, but so far, the russians seem to avoid drafting women.
https://www.pewresearch.org/fact-tank/2015/08/14/why-the-former-ussr-has-far-fewer-men-than-women/#:~:text=The gender ratio in Russia,Soviet nations are similarly low.
that's roughly 50 million men.
https://www.statista.com/statistics/1005416/population-russia-gender-age-group/
if you ad the columns from age 15 all the way through age 65, for men, you get 35.4 million.
That certainly seems like a sufficient number, but you have to take into account that some are the children of influential parents, some have handicaps that prevent them from serving, some will just refuse to go, some will be involved in critical industries, ...but even taking all of that into account, they should still have 25 million men of serving age.
The problems seems to be finding them adequate gear to fight with, taking the time to give them training of any kind, and the unsuitability of the command structure of the russian army.
https://www.statista.com/statistics/1006655/ukraine-population-by-age-group/
it looks like Ukraine has about 20-25 million adults of fighting age, but as they will have roughly the same percentage that can't/won't fight, and a percentage of their women who can't fight, call that 16-18 million, but 16-18 million that have good gear, decent training, in a better command structure, with no shortage of weapons, no real shortage of ammunition, more modern, better maintained weapons, better medical care, and usually better intell.
They're also defending their homes, on territory many of them are familiar with.
I can't see the russians being able to sustain their assaults for long before they'll have to start rounding up men in the streets and forcing them to fight at spear point, as bullets will be too scarce to waste on deserters...It's all good for the russians to say they support putin on camera, but will they support him with a rusty rifle and a handful of ammunition, wearing worn out shit that smells like mold, and doesn't keep them warm, knowing that they're going to die whether they advance or retreat?