Believe it or not, but the number in parentheses actually is the number of unread messages in that folder. Maybe it also aggregates the number of unread messages from subfolders, if you happen to have those.

To check if you have unread messages, you can set a filter to only show unread messages by going to the menu with Alt or F10 and then choosing View > Conversations > Unread.

Remember to switch the view back to All once your done to show all messages again.

If you need to switch often between displaying all/unread messages, it can be handy to add the View: control to your Mail toolbar: Right click on the Mail toolbar and select Customize… Now drag the View: [All] item to the Mail toolbar. This allows you to easily switch between viewing all/unread messages.

With the filter set to unread and no unread messages, there is still a number at the inbox.

I noticed something that may be affecting that number next to my inbox.

I just tried to create a new subfolder, but a dialog box indicates that it already exists. However, it is not listed in the panel. I don't know how to retrieve it. This may be the number of messages in the number above.