How to increase the maximum number of opened editors in IntelliJ?

Same as best answer but with snapshots!! (windows 10 with IntelliJ Idea 2018.2.4):

Settings | Editor | General | Editor Tabs | Tab limit.

You may also want to increase the recent files limit in Settings | Editor | General | Recent files limit if you are using the Recent Files (Cmd+E) feature.

Works for all IntelliJ IDEA platform based IDEs.