Russia never had a manpower problem, it had a political willpower question.
Why limit to only 200K soldiers in Donbass? It's a political issue (i.e. "SMO"), not a population issue.
The Russians cannot keep a large army in peacetime due to the size of its economy. Once the war started, they are doing a sort of covert mobilization. There are a lot of volunteers and they were talking of needing an additional 200K troops by signing up the volunteers. The idea is that these new troops will take over non-combat duties like securing areas already taken, freeing the current troops for the tip of the spear. Additional troops do not change the way they fight the war much for the following reasons,
1. There is no urgency since they understand that NATO will not send troops. They can take their time.
2. A long war will allow the Russians to destroy infrastructure and in general break the morale of the Ukrainian population. Only when the Ukrainians understand that NATO is impotent to help would they have a change of mind.
3. There is another limitation to the Russian arm forces. In peacetime, they don't have the money to have a large logistics arm to supply the troops they already have. Throwing 3x the number of troops into combat would hugely magnify this problem.
4. Fighting a slow war in their turf would increasing the problems of logistics for the Ukrainian army, which already has a big issue with them. This would allow the Russians to destroy the Ukrainian arm forces much easier in this war of annihilation.