Hello Linqen, thank you for posting your question.
To disable One File per Actor:
From the main menu, go to Edit > Editor Preferences.
From the sidebar, select Experimental.
Locate the Tools section and enable the check box for One File Per Actor.
You can enable it on a per level basis but from what I can tell, you’d need OFPA active.
If you would like to learn more, here is the documentation on OPFA.
I hope this helps you.