I've seen a couple of analyses that say of of those 12,000+ tanks in the Russian fleet?
Only 4,000 to 5,000 of them were combat ready BEFORE the Ukraine incursion, and it's even fewer than that now.
I've also seen some write ups that feel that the T-14 Armata, or whatever its designation is, is in the top tier of most capable battle tanks in the world today, easily in the same class as the Leopard 2, Challenger 2, or the Abrams 2. Those were also written before the Ukraine incursion. It's a pretty good bet that while the tank may be capable, the Russians manning the things... aren't.
Whoops... NOT the Armata, which hasn't entered service yet... the T-90.