i was re reading your questoin
Okay there are some corrections
is not 254 per SSID its 254 per AP.
In a single cluster there can be up to 2000 devices with the lastest firmware as fari know. Is not a hard limit but is recommended.
Just in AP 9x model the cluster is limited to 254 devices per cluster, but your AP is a 225 so you should be good to go!, you got the AP with the best CPU RAM of all Aruba APs.
What i show you before is how many client devices can be in one AP(Not per SSID, per AP! okay? )
Cheers
Carlos