you can't increase the number unless you compile the source code

This is certainly a problem to me: the list under "Go" to "Folder" to "Recent" changes with great idiosyncrasy. A mailbox that I seldom use stays on the list, but one that I often use disappears from it, and no means is afforded to drive it in. If it were "View" to "Folders" to "Recent" it would be easy. What makes a mailbox thus "recent"? How do I keep it "recent"?