How many can run at once
MSL runs at most four instances together, and why suspended ones count.
On this page 3 sections
MSL runs at most four instances at once. The Concurrency card on the Overview tab shows "Running now: n of 4", and the capacity pill at the top of the sidebar shows the same, turning orange when all four are in use.
Why there's a limit#
Every running or suspended instance keeps its memory on your Mac, and virtual machine memory can't be paged out. Without a limit, it would be easy to start enough instances to leave macOS gasping. The default memory size is chosen so that four instances together stay well within your Mac.
What counts#
| State | Uses a slot? |
|---|---|
| Running | Yes |
| Suspended | Yes — it's frozen, but its memory is still held. |
| Hibernated or stopped | No |
When it's full#
Start is greyed out on stopped instances, and trying anyway says "4 instances are already running … Suspend or shut one down first." Things that would start an instance — Find Apps, a sharper icon for a Dock tile — say the same, or quietly fall back (an app added while the slots are full keeps its existing icon).
To free a slot, hibernate or shut down something. Suspending doesn't free one. See Suspend, hibernate or shut down?.