How many concurrent connections do the express editions allow?
My front end uses standard ADO.Net code where I open the connection to the server, get my data, and then close the connection. Am I right in saying that as soon as the connection is closed, it then allows this connection to be opened by another user?
The express editions of SQL Server don't cap the number of concurrent connections - they exert limitations in other ways - such as the maximum size of the database (4GB), CPU sockets (1) and amount of memory (1GB).
More info here.
You are right in saying that when a connection is closed, its resources are released immediately. The only caveat on this is connection pooling in .NET.
.net handles all of that for you. It creates a connection pool per unique connection string and your DB calls will share the connection. .Net does not really open/close real connections when you call conn.Open() , the connection pooling handles that.