Per the latest (Feb. '23) edition of The Military Balance, Russia has 2,070 MBTs in service (~2,300 with those of the DPR/LPR), including damaged vehicles.
According to the same source, Russia has a further 5,000 MBTs in store that could (eventually) be refurbished, ranging from old T-62Ms to quite recent T-90As.
Meanwhile, Ukraine has ~1,000 MBTs in service, again including damaged vehicles. About 1/3 of Ukraine's tank park is made up of Russian trophies, however, whose actual serviceability is dubious. The Military Balance doesn't list any Ukrainian MBTs in long-term storage.
All in all, the Ukrainian ZSU probably can muster at most a few hundred MBTs at the front at any given time, whereas I'd be impressed if the Russian VSRF can field 1,000 at the moment.
Russian MBT total losses since November appear to amount to ~100 per month. They were much higher during Ukraine's successful counteroffensives of September - October, or during the initial phase of the invasion last winter. Since Russia renewed major offensive operations over the last few weeks, their MBT losses haven't significantly increased.
Meanwhile, Russian manpower losses have been steadily increasing month-on-month since the beginning of mobilization in late September. January saw the worst Russian personnel losses of the war so far, and February is on track to surpass it.
Maybe the Russians are trying to conserve / rehabilitate armor by throwing a wall of men at the ZSU?