This article describes about SSIDs per AP Profile.
Solution
At a given point of time only 8 SSIDs can be added per radio per AP profile.
A 9th SSID is greyed out and cannot be chosen as shown in the screenshots.
A second radio with another 8 SSIDs can be added to the same AP profile if needed.
Ideally if users add 8 SSIDs in one antenna, then the same SSID should be added to the other antenna as well.
All the SSIDs should be added to both 2.4ghz and 5ghz antenna as part of best practice.
2.4ghz offers less speed, 5.0ghz offers more speed. Hence it is recommended to add the SSIDs to both the antennas.