Dude. Why *wouldn't* they use plentiful North Korean cannon-fodder instead of their own guys, if it's on offer?
That doesn't mean there aren't a shitload more Russian men they can throw into this thing. There are, Russia's barely really mobilized much at all for this conflict. They're not even on a real full-on war footing. If they have to, they can. Thing is, NK and China cozying up to them lately is super fruitful - if Kim's willing to show his loyalty to the little bald karate-and-judo-and-shirtless-horse-ridey guy by offering up combat virgins as artillery cover by the thousands, of course Putin's going to welcome that. Easiest solution, you know the NK fat-man is serious about the alliance, and you don't waste any of your own guys in case you need them down the line.
Russia has limits, absolutely. They're just nowhere even close to approaching them, fighting...fucking Ukraine of all places. If this spirals into Russia V.S. Poland or whatever (even without a full NATO response as a whole), then you'd likely see Russia rounding up a majority of guys between 18-50 to throw at the fight. They...haven't been doing that with Ukraine, like at all. They're just burning through their criminals and sending whoever's willing to volunteer for combat, there's no major-level draft.
Think the last numbers were 300 000 mobilized, and most of that is supply-chain, only volunteers going into combat. Supplemented by the get-out-of-jail-free convicts choosing likely-death over continued incarceration. Most analysts estimate the entirety of the Rooskie armed forces (active and reserves totaled, draft enacted, everyone mobilized to go) as at *least* 2 million, probably closer to 3. Don't kid yourself, if this was Russia at full mobilization from the start this thing wouldn't have gone the route it has with this quagmire/near-stalemate. The reason it has, the reason the Ukrainians have done better than epected since the start, is because Russia dipped a toe into Ukraine, they didn't jump all-in with everything they had.