If you have reached the limit of active monitors for your subscription, then you will not be able to add any new monitors. Just upgrade your subscription with the new number of monitors you want to operate. Any price difference caused by upgrading or downgrading will be automatically calculated to make sure you don't pay more than you should.
New monitors will appear as inactive ("Initialized") in your account until the player gets connected for the first time and registers successfully. You can manage inactive monitors just like registered ones, they just have not been paired with an actual player.