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%';