These are some limits found in the 2007 version of SharePoint, specifically Windows SharePoint Services 3.0.
Guidelines for acceptable performance
Scope of impact when performance degrades
50,000 per content database
Total farm throughput degrades as the number of site collections increases.
150,000 per Web application
This limit is theoretical, and is dependent largely upon:
Performance of the database server on which the configuration database resides.
Performance of the Web servers in the farm.
Network bandwidth between the Web servers and the database server.
This is not a hard limit, and assumes a single database server. Your environment may not be able to host this many site collections per Web application. Distributing content databases across additional database servers can increase the effective limit of the number of site collections per Web application. You should perform testing to determine the actual effective limit in your environment.