Limit number of blog entries on first page

There are now 15 blog entries, the latest 12 of them appear on the main page, the other ones are accessible via a link at the bottom “Altere Artikel” ( = older entries )

Very nice, but: How can I set the limit elsewhere, having less entries on the main page, and more moved to the history part ?

The page I need help with: [log in to see the link]