At the Amazon RDS FAQ there is the question "What is a database instance (DB Instance)?".
The entire answer (as of mid-June 2012) is:
You can think of a DB Instance as a database environment in the cloud
with the compute and storage resources you specify. You can create and
delete DB Instances, define/refine infrastructure attributes of your
DB Instance(s), and control access and security via the AWS Management
Console, Amazon RDS APIs, and Command Line Tools. Multiple MySQL
databases or SQL Server databases (up to 30) or Oracle database
schemas can be created on a given DB Instance.
The last part of that quote, "Multiple MySQL databases or SQL Server databases (up to 30) Oracle database schemas" I interpret to mean that you can have an "unlimited" number of databases on an RDS MySQL or Oracle instance but only 30 databases on an MS SQL Server instance ("unlimited" meaning not limited by the RDS infrastructure itself).
This was asked in the Stackoverflow question Does Amazon RDS support multiple databases per instance?. The answer quoted an older version of the FAQ.
What I am looking for is an Amazon document that clarifies this question, or else someone who has experience using Amazon RDS who can attest what the situation actually is.
Best Answer
As an AWS customer using MSSQL in RDS, I can confirm the maximum amount of databases per instance (t1.micro) using SQL Express is indeed 30.
I am testing other instance sizes, and will post further if I get different results.