Just to clarify - is your expected behavior that the editor will insert a line prefixed with '2. ' when you hit enter after the first line?
If so, that is not possible with the Markdown editor. The Markdown editor is a plaintext editor that won't auto-complete formatting. You can try out our new rich text editor (currently only available on pull requests) - which has this feature - by opting-in to the Atlassian Editor feature on your Bitbucket Labs page: https://bitbucket.org/account/admin/features