On a Sandvox Weblog Collection page, it is possible to control how many posts are shown on the page using the Inspector. You have to click the little drop-down arrowhead to show the --> Items option (blank means Show All) (click to see picture)

My example site: http://www.isawhow.com/_tests/rss-feed/ 
(click to see site)

Sandvox 1.6.1 does now control the number of posts like the syndication feeds option of the WordPress Settings panel as show below: