number of mail accounts visible in left column - how to increase?

number of mail accounts visible in left column - how to increase?

In addition to the zoho.com account, I have 4 POP accounts configured in mail. In the left column, only 3 of those are shown. The ones not shown have to be selected at the bottom by clicking on the "more" button. Is there a way of showing all accounts?  Also, is there a way of showing the unread mails of all accounts simultaneously?

Achim