Hi Dewang,
As of now, we don't have a command to give the number of active applications. You can check the threadlist command output. It can give you an idea on busy the server is based on number of DM threads.
[Thread group=DE_INSTANCE_NAME; active threads=10]
DM [3.127 seconds]
DM.dm-appl-builder [0.496 seconds]
DM.dm-timer-pop [29.154 seconds]
DM.WAInputSessionRequestProcessor:7507 [5.193 seconds]
DM.AgentHeartBeat [1.222 seconds]
.....
However, the exact numbers can be seen on the Desktop Client, under Monitor -> Dashboard -> Summary Dashboard.
Thank you,
Nitin Pande
Broadcom