You can have as many databases per instance as you like. Licensing is either priced per CPU (approx $6000 per CPU for Standard, $25,000 for Enterprise on SQL 2005) or server license + cost per user/per device license. Take a look at http://www.microsoft.com/sql/howtobuy/default.mspx. It's got lots of good info to help you decide which edition of SQL you'll need and the licensing options and US costs. I found this really useful.

BTW, if you are in the UK don't expect to take advantage of the great dollar rate and expect to get your license for around £3000, as I recently found out. You'll pay over £4000 in the UK for a license that costs $6000 in the US!

Julia