It’s an arbitrary limit imposed by Apple’s licence agreement for macOS, which goes right back to Mac OS X Lion in 2011, if not before. The moment that you try to run a third macOS guest, the Virtualization framework returns an error and fails to start it, with a VZErrorDomain Code of 6, interpreted as “The maximum supported number of active virtual machines has been reached.” That doesn’t appear to be a hardware limit, as an Ultra chip can afford to give each of the VMs 4 vCPUs and 8 GB of memory and still have plenty to spare for the host. The limit on the number of macOS guests is more significant, though.

Although it’s a little disappointing that it doesn’t work at present, I don’t see anyone losing sleep over that, and can’t envisage it constraining what we can do with lightweight virtualisation. Nesting is an issue of more theoretical than practical importance. Even with the full might of a Studio M1 Ultra and 128 GB of memory, you’ll have discovered that you can only run two macOS guests at once, and they can’t be nested. If you’ve experimented with lightweight virtualisation of macOS on an Apple silicon Mac, you may have tried a couple of tests, to see how many virtual machines (VMs) it can run simultaneously, and whether they can be nested to run a macOS guest inside a macOS VM.

Utm use apple virtualization