The Sample Database is installed on each workstation, so logging into the Sample Database does not take up a user license on the network.
For Blackbaud products using SQL Server:
The Sample Database is attached to the Blackbaud Management Console on the server, so logging into the Sample Database takes up a SQL Server MSDE license. MSDE has a limit if 10 concurrent licenses to the database. SQL Server Enterprise (or other non-MSDE versions) do not have this limitation.
Also refer to What is the difference between SQL Server 2000 MSDE, Standard, and Enterprise editions? (BB95014)