There is no strict upper limit of APs in an Instant cluster and it depends on the number of clients, traffic patterns, VLAN usage, and others if you will have proper performance.
The general guideline for Instant is not to exceed 128 APs in a cluster or 2000 concurrent clients.
You can also split your deployment in multiple clusters and configure L3 roaming between the clusters.
With more APs in the cluster, you will need a better design of your deployment. It may be good to consult an Aruba partner with experience in this kind of design.