It’s very ducking complicated, but I’ll do my best to give you a sensible answer. I live in Russia and while I’m no journalist or expert, maybe I have something worthwhile to say for an insight.
We do have the numbers, period - there’s money in killing our neighbors, there’s some sort of twisted fate or purpose that always emerges during this kind of times, and there’s people willing to do this kind of stuff for the kind of money or purpose offered. There’s also, well, just people of various backgrounds, skills, and capabilites to forcefully throw into the war effort, but the most important thing is that it’s not just a number game - like, it’s not a dead-simple RTS game where you select some units and magically convert them into equally capable combatants over a set period of time to go and win with some tactics.
Despite the somewhat prevalent opinion, this is not a popular war, it’s not supported or sacred or anything - Russia wouldn’t see so many people fleeing and imprisoned otherwise. Wouldn’t have to forcefully mobilise anyone either.
There’s enough people in the country that the government can try and throw at the wall of this war and see if they stick and magically do something, but that doesn’t guarantee any success of its own and has massive risks that even the current old men aren’t willing to take.
As a bonus, any good dictator loves a war, especially a war that’s prolonged, that’s convenient excuse for anything - establish the right kind of info, punish anyone who disagrees, make people praise you for the very little they may get because things could always be worse, make the war the excuse, tell people it’s good and creates work places and gives them purposes, and so and so forth. I don’t belive Putin wants an end to this war - he’d much rather let it help him sit tighter on his blood-drenched throne, and make Ukraine suffer for not playing along with his egomaniac ambitions; under Putin, the war dies with him, not a minute earlier.