In my opinion, part of being a good SharePoint admin is to pay attention to the size of your content databases and try to keep them to a specified maximum size. (Usually for DR reasons) In order to do this, you should limit the number of site collections per content database. Naturally, site collection sizes will vary, so setting a generic maximum number might not be ideal. There are two ways to control the number of site collections in a content database. The first is to set the status of the content database to Offline. While this is a novel idea, doing this can cause some issues. One of the biggest I've seen is issues with different backup products. A better way to limit the number is to change the maximum number of sites for the content database to the number of sites in the database when the database reaches the desired size. You may need to lower the number of sites that will generate a warning, but there's no control on what this number actually is. Happy Admin'ing!