Figures I've seen vary, on various expert sites, but broadly surround[1] the current wikipedia assessment (either common consent, or the strongest opinion of the very most recent editors suggest, I didn't check the respective page histories) which says that there is a full Russian possession of 2,030 T-72s active (8k 'stored'[2]), 480 T-80s (3k 'stored'), 417 T-90s (+200) and the possibility of a score or so of the newest-gen T-14 (which was supposed to be another full 2,030 delivered by 2020, but got seriously paused five years back and it now I think they're regretting not sacrificing some other bit of the national budget).
Clearly, Russia won't deplete the whole rest of its borders (NATO or non-NATO) to supply AFVs to the theatre, "in case of counter-invasion"... They say they fear NATO attack, and if I can't say for sure NATO would not if they left skeleton forces (or less) there. And I also can't guarantee the 'goodwill' of China or other opportunists. And more strategic reserves must remain 'in-country' ready to patch up those forces (or multiply the total additional deterant factor by sheer flexibility) covering whole swathes of borders with multiple hub-connected. So before they get to reassign any significant fraction of the massive total[3], l they're going to have to bite the metaphorical bullet and move onto some other phase of conflict where tanks are getting moot (or at least need their NBC protection to work well).
(Russia also needs to keep Russia/Occupied-Areas borders gunned up, in case of rapid collapse, change-of-allegience 'occupiers', any sneaky deep-penetrators, dealing with spontaneous fifth²-columns no longer sure that Russia is the answer, etc.)
Guesses as to how much they could/would field to Ukraine vary wildly. Ukraine has the 'advantage' of basically having a lot more front line, in proportion, to the total of not-currently-frontline, not-likely-frontline, frontline-only-against-allies and area coverage they need to consider maintaining response forces (for external or internal reactions) dotted around, just in case. And with the promised numbers of Generation 3, 3.5 (i.e. enhanced 3rds) and possibly 4, competing with the respective numbers across the Russian spectrum (some upgraded T-72s may be considered 3rds, but may be under the T-90 count; T-80s could be seventies 'early 3rds' from Soviet era or nineties enhanced models more realistically 3rd) seems to be one of the interesting factors. Of course, the inferior stock has already been depleted by Ukraine's initial resistance (including local-adaptation Ts that seem to be better/more available than similar late-variation Russian homegrown forks of the product line managed to be).
I think the stated numbers of required tanks (not bleeding-edge, but mostly not junk) is going to have a disproportionate advantage over the Russian stuff (fixer-uppers and hastily upgraded where so done, as a dominant trend). But this is something you need to refer to somewhere like Jane's, though, or be a good analyst your own right if you don't have one of them in your pocket, willing to whisper into your ear as well as the ears of the Agency they should probably be working for...
(Belarus adds a few hundred more, mostly '72s, if they can be pursuaded to do anything. Would be interesting to see how Russia is working towards that, but surely needs the 2023 Spring Offensive to go really well for Russia to give their ally the incentive to narrow the other grip on the vice. And, before that, support for Ukraine is going to have to tip over into either far greater or far less than now, to change the whole dynamic.)
[1] Some could be better updated, having removed those known as no longer in use due to such as having suffered indegestion from eating too much many bits of non-Russian metal...
[2] I honour the term used, but this figure may total more than they are theoretical capable of actually unmothballing/whatever, never mind those that they are actually seriously trying to bring to service.
[3] They could 'afford' to near clean out all training areas of tanks, of course. And might do the WW2 thing of driving the vehicles off the Tankograd production lines straight towards the battlezone[4], eschewing the break-in testing phase.
[4] Ok, so unless they're really running short of conscripts, they won't have their final fitters-and-welders become their first (and 'virgin') crew, thowing them into the battle just as desperately.