As with almost everything in this war, there are no 100% concrete proof
But, think about it a bit. Russia has for centuries been plagued by corruption and embezzlement, especially during Soviet time. Tho, during this time they had the military spending and GDP to keep up with it. Soviet Union collapsed, Russia become a kleptocratic disaster for atleast 10 years and then they have slowly been building up, althought still being a corrupt kleptocracy from within. Keeping up with nukes and all those reserves would cost an immense amount of money, something that Russias military spending doesnt account for, especially when including corruption in the mix. There are also several pictures of these areas with old reserve tanks, most of them are rotting. At best, they can be used for spare parts to repair and restore older tanks. Russias ability to produce modern tanks with modern systems is limited, especially with new sanctions
Of course, this is not absolute truth. Just speculation based on tons of pictures, reports and theory. Its all up to you if you want to believe in Russian supremacy or not
Look at which vehicles are used most in the Ukraine war, from both sides its mostly tanks originating from the 70s and earlier, same with other armored vehicles. Russias armored fleet doesnt consist of modern vehicles, but mostly 30-40 year old systems or said systems but upgraded a bit. Its a delayed cold war galore, took 30 years but its here.
But still, tactics and training has a much larger impact. If the west throws hundreds of modern tanks, they surely will be much better than what russia is countering with but would still take big losses. As arabs cant fight, several modern western tanks got BTFO'd by tank-killer teams. Although, western tanks have much higher potential when coupled with good training.
Honestly, in the current numbers i also think that the tanks supplied by the west are overrated. Western tanks are absolutely superior, but only when coupled with good doctrine and air support. Ukraine has only some of the doctrine and none the the air support. Would be better to just buy hundreds of cheap older tanks and slap a bunch of modern sensors and reactive armor on it, coupled with modern IFVs