Numbers opens new files as tabs instead of separate windows

Hi, I think I can help! Go to System Settings > Desktop & Dock > Scroll to the "Windows" section, then the first option "Prefer tabs when opening documents". See what that says and try playing with the setting to see if you get the behavior back to how you want it.

Numbers does have the ability to display documents as tabs rather than separate windows, but I can't see any way to make this the default - usually it's the result of a specific user action (namely Window -> Merge All Windows, which combines all open windows into tabs in a single window).

However, even if merged, newly opened documents still open in their own window, at least on my machine.

Oddly, there is no un-merge menu option, but you can drag each tab off the tab bar to move that document to its own window.

I can't find any state where documents open in tabs by default, though.

Oh, nice. I didn't think about a system-wide setting. Since the OP only mentioned Numbers I didn't think past that, and this setting should affect other applications, too.