Russia does run out of tanks, gradually. That's why it has started to use obsolete Soviet-era tanks. The day when Russia completely runs out of tanks isn't anytime soon, since Russia has thousands of obsolete tanks in storage.

Still, the number of tanks Russia has diminishes every day, and their quality is worsening. In the meantime, Ukraine gets new rather modern vehicles from the West and captures a lot of Russian ones. Having an advantage, Ukraine is going to destroy even more Ruscist tanks - and thus move closer to the distant victory.