enigma1 - 3:56 pm on Feb 14, 2012 (gmt 0)

It also depends how the database is configured. There are settings to specify the maximum number of simultaneous client connections. Then it's the site application code, there are ways to keep track of clients and save resources and you can also check the database activity before it gets out of control and breaks. So different sites with identical hardware specs can handle a totally different number of visitors.