I'm not aware of any practical limits on device numbers in central. Note that when deploying Aruba Instant APs via Central, there are design constraints on cluster sizing. You should follow the Aruba Instant VRD for guidance on the low-level design, especially if you exceed 15-25 APs in a cluster.
If you think that you have a design with thousands of AP's in central, it may make sense to contact your local Aruba SE.