Is there an option to display the number of emails in a folder in the folder list pane?
Not in the core install, no. But since you can see the total count in the Folder management page, I'm sure it could be done with a plugin. I don't see that such a plugin currently exists, so someone would need to code one.
Side note, plugin repository is located at http://trac.roundcube.net/wiki/Plugin_Repository