If you want the list of databases currently being used / active connections

 SELECT 
    DB_NAME(database_id) AS DatabaseName,
    COUNT(*) AS ActiveConnections
FROM sys.dm_exec_sessions
WHERE database_id > 0
GROUP BY database_id
ORDER BY ActiveConnections DESC;

If you want BOTH list of customer DBs + active connections
SELECT 
    d.name AS DatabaseName,
    ISNULL(c.ActiveConnections, 0) AS ActiveConnections
FROM sys.databases d
LEFT JOIN (
    SELECT database_id, COUNT(*) AS ActiveConnections
    FROM sys.dm_exec_sessions
    GROUP BY database_id
) c ON d.database_id = c.database_id

WHERE d.name LIKE 'C%';