Hi,
With IAP deployment, concurrent users are depends on how many AP are members of that cluster hence it highly scalable.
in IAP deployment, client traffic will be handled by the local IAP itself. but if you have local DHCP(DHCP in VC) model definitely there will be a cap for the pool size. hence Aruba came out with a deployment model called Distributed L2 and Distributed L3, in these model DHCP will be running in the respective IAP rather than in centralised VC.
Therefore, with Aruba IAP you can have highly scalable deployment model interns of concurrent clients and DHCP.
hope got some clarity on this,
Please feel free if any further clarification required.