Like the AOS Controllers IAP also supports Dynamic Multicast Optimization and Multicast Transmission Optimization.
There is no hard limit on IAP cluster size except for the IAP-9x which is limited to 16 per cluster. Also, if you introduce an IAP-9x to a cluster with other models, such as the IAP-105, that cluster becomes limited to 16 IAPs (lowest common denominator). There are a number of threads on this topic. While we have customers running large clusters (150 -200) generally clusters are smaller based on geography. The nice thing is that you can manage multiple clusters via Aruba Central or AirWave.