It is possible to run macOS on beefier hardware (if you ignore the license) and run more VMs per host (again, if you ignore the license). In Apple's case, they

It is possible to run macOS on beefier hardware (if you ignore the license) and run more VMs per host (again, if you ignore the license). In Apple's case, they don't have license issues to worry about and thus can do their SaaS/CI stuff in VMs on as many of the beefiest servers that make sense to buy instead of having to fight with Mini's or racking Mac Pros. I expect that most likely they are currently doing that on beefy machines from a cloud hardware vendor like Quantas or Dell.