If its for the entire instance, then Alerts, and define a SQL Server Performance Condition Alert on SQLServer:General Statistics, Logical Connections.

For a specific database, maybe something along the lines of scheduling:

select db_name(dbid), count(dbid) from sysprocesses group by DB_NAME(dbid), throw the output into a table and query it with a RAISEERROR or send dbmail to let you know that way?

Cheers