Enough to pay the salaries of those keeping them on life support, more like. I wouldn’t say they are going anywhere. To be clear, that doesn’t mean they are going to die anytime soon. Mainframes are still around and getting refreshed and upgraded, too.
However Aix, Solaris (and Open Solaris derivatives), z/OS, IBM i (AS/400), ClearPath MCP, OS 2200, are still being updated and sold.
That list is not only UNIX systems.