$cat problem

I read that if I wanted a page to display only posts from certain category(ies) I should use the $cat = x,y,z,etc; line to show posts from categories x, y, z, etc… I tried to do this, but upon doing so, the page shows every single posts from those categories, instead of the latest number of posts from those categories I specified in the ‘Show the most recent # posts’ in the WordPress Options > Reading page. How do I set up the page so that it only shows those number of posts from those certain categories?