Microsoft has come up with the following benchmarks, beyond which performance suffers.
|Site Collection in Single Content db||10000||15000|
Navigate to Central Administration and then Application Management
Central Administration> Application Management
Click on Manage Content databases Under Databases
Modify the Maximum Number of Site per Content db. (15000 to 10000)
According to Microsoft, you can have up to 50,000 Site Collections in a single web application. The bigger question I think you should be asking yourself is, how many Site Collections do you want to put in any single content database? When you look at SharePoint from a recovery perspective it is important to limit the size of your content databases so that you can manage your backups and restores. Each Site Collection can only reside in a single content database, though you can have any number of collections in a content db. So, while I can have 50,000 collections in my web app, how many content databases will that be and how many of those collections will be in each db?