You could use "show" commands for this.
For example "{HOST}/v1/configuration/showcommand?json=1&command=show+global-user-table+count&UIDARUBA=XXX" should give you the number of clients. This does not work because it count every client with every IP (so it seems).
What I do is to use "show global-user-table list" and then count the unique MAC adresses.
Hope this helps you. If someone has a better soloution, please let us know!