From Secrets of SharePoint – Tip of the Day
Mattias Karlsson – When creating a new content database in SharePoint 2010 the default value for maximum amount of sites is set to 15,000. A quite ridiculous figure since the soft recommendation from Microsoft for a content database is 200 GB. You should always create Site quotas and then calculate how many sites you can have in each content database to control storage in you content database. But even if the soft recommendation is as much as 200 GB, you should align the number of sites and the amount of storage in each content database so you can fulfill the requirements on disaster and recovery.