Hi Wilfredo,
The best option to get the devices on Central is by adding the master IAP from the cluster using the cloud activation key & it will populate the list of the slave IAP's on Central as well.
In case you would like to use the option of adding the device using mac-address/serial number, we can only add up to 32 devices by default.
You will have to contact TAC in order to get the device quota increased on your Central Account to more than 32 devices.