If Mac desktops keep changing order, turn off Automatically rearrange Spaces based on most recent use in System Settings under Desktop & Dock, in the Mission Control section. Then verify the change by switching among several desktops and reopening Mission Control.
Change the specific setting
Open System Settings, choose Desktop & Dock, scroll to Mission Control, and disable the automatic rearrangement option. This setting concerns the order of additional desktops according to recent use; it does not name Spaces or pin individual windows by project.
Use Control-Left Arrow and Control-Right Arrow or trackpad gestures to confirm that your expected left-to-right order remains usable.
Test the whole navigation context
Include an ordinary desktop, a full-screen window, and each connected display in the check. “Displays have separate Spaces” changes the multi-display model, so record whether it is enabled. Avoid changing both options at once because you will not know which one fixed the behavior.
Use stable clues beyond position
Even with automatic rearrangement disabled, application switching may move you to a Space containing that app’s windows if the corresponding setting is enabled. Full-screen areas can also affect the visible sequence.
Use distinct wallpapers, recognizable window titles, or project names as secondary clues. A numbered habit such as “Desktop 3 is always client work” is fragile if other Spaces are added or removed. The setting can stabilize order, but it does not turn desktop position into a permanent project identifier.