Hi , you can only have a maximum of 2 public keys at a time. To add a new one, make sure your application is using one of the 2 keys only, then delete the other one and create a new one.
We only support 2 keys because that is all you need to swap out one key for another. You would create a second key, swap your application over from the first to the second key, and then when all traffic is using this new key you delete the first one.