1. Russia makes almost no industrial parts itself. They don't have the know-how or machinery to do so, because it has atrophied from unuse over the last thirty years. Most industrial parts come from Europe. Europe has banned exporting industrial parts to Russia. Therefore Russia can not make more of its current tank types (or anything really). They'd have to reboot all their industry from scratch to be able to build from scratch. A decades long project at least.

2. Most of the reserve of russian tanks are in long term storage. They are reportedly in complete disrepair and far from combat ready. Russia doesn't have the industrial capacity to repair them, see point 1.

Bottom line is that the tanks Russia has committed to Ukraine are it. Ukrainians are blowing them up at an alarming rate, and Russia has only weeks worth of tank supply left before they are at a direct disadvantage in tank supply compared to the Ukrainians who are gaining supply.