This article applies to mixed IAP cluster with at least one IAP 92 or 93 in the cluster.
Initially, when Instant APs were introduced, there was a limitation where we could add a maximum of 16 IAPs in the cluster. This limitation was enhanced in the subsequent codes and eventually with the new versions like 3.1, 3.2 and 3.3; we can have any number of IAPs in a single cluster.
However, important to note is as of version 18.104.22.168-22.214.171.124, this applies to all the IAP models except for IAP 92 and 93. The maximum number of IAPs in a cluster having IAP 92/93 is still 16.
So in case the code allows more than 16 IAPs in a cluster but you are unable to bring up the 17th IAP, it is most likely there is at least one IAP 92/93 in the cluster. As soon as you remove those IAPs (92/93), you would be able to add more IAPs in the cluster.