There's no certain numbers and I doubt that even russians themselves know for sure. Others have commented decent quesses, but if wikipedia has decent data they had about 14000 tanks in various conditions when they started the attack. Majority of those are WW2 era soviet stuff and for the condition they're in you can just throw a dice, that should be about as accurate as any other estimation.

They've been hauling old soviet stock back from pretty much everywhere and manufacturing/buying new ones as fast as possible, but my personal guess is that they're pretty much sweeping every corner to gather enough working components to hack together something that even moves.

I don't have anything to back this claim, but for a while now the russians have been more and more willing to negotiate which, to me, says that they're running out of hardware pretty soon, but not just quite yet.