simple catch archive

simple catch www.consructionsoftwarenews.net ok this might be a dumb question but here goes i set up a drop down archive botom right no problem. ok can i control the number of post before they go to archive ? at the end of the month do they drop off post and are in archive ? as I add post does the page just keep growing i want to be able to set the number and if a new post is created the last on goes to archive