Mailchimp listing only one RSS-Item

Try adding a pubDate -- publication date -- to your items. This is an optional item for RSS, so it's valid without, but Mailchimp requires it, I believe.

If you're coding the feed yourself in PHP, bear in mind that PHP's date formatting constants helpfully include DATE_RSS/DateTime::RSS.

Mailchimp also has an example RSS feed shown in one of their Support articles -- if you're still having problems, compare your feed to that example.

Also, note (from here):

When you first set up an RSS campaign we will show the last post as an example if you haven't added an article in a few days. If this is your first send, we'll only send the posts from 24 hours before your campaign was activated for daily, from the last 7 days for weekly, and the past 30 days for monthly.

So I'm fairly sure that's why one of your posts is showing up -- because there's no publication date on any of them, Mailchip is just showing you the first one it finds as an example.

Another thing to watch for is if the feed has been run before. e.g a daily send going out at 9 am.

If you preview an edit to the design it will only show new stories that haven't been sent out.