The maximum number of monitored lines is 50 but you should consider bandwidth. The maximum may also be determined by the number of line buttons on the user's phone.
From the customer view in https://admin.webex.com, go to Users, and select the user you want to modify.
Select Calling, choose Advanced Call Settings, and then go to Monitoring.
Choose whether you want this user to be notified about parked calls, search for the person to be monitored, and then click Save.