You only pay for licenses over 4 OS and 2 SQL Server (or outright new ones) after graduating BizSpark. 2 SQL Servers (OS licenses are chump change by comparison) go a loooong way* too, as you're presumably scaling up rather than out.

http://www.microsoft.com/bizspark/about/Graduation.aspx

Through the grapevine I hear the "review" process (for more free licenses at graduation) is extraordinarily accommodating, though I'd guess if you're spinning up a box per customer or something insane you'd get denied.

Lots of people get this wrong, I'm surprised Microsoft hasn't carved this correction into a mountain somewhere...

*Stack Exchange is running 2 + failover ( http://blog.serverfault.com/2011/09/30/the-stack-exchange-ar... ), and was running on 1 not too terribly long ago.

Did not realize it changed later. Thank you.