In general, the IAPs have no pre-defined limit in firmware.
If you are using the majority of the common IAPs(105, 115, 135, soon to be 225) you can easily scale to dozens of IAPs in a single cluster. This enables you to cover a significant area in a building or campus.
You can have multiple buildings/campus locations covered with seperate clusters to scale your network as larger as you need.
Remember, all IAPs in the same cluster are in the same L2 domain, so common/traditional best practices apply ... e.g you can't make that L2 domain infinately large...without consequences.
I typically deploy IAPs when I have a building that needs 10,20,30,40,50 APs. Works well every time.
JF