If you have >15 packages mounted, you need to add a second SCSI Controller to mount another 15 packages.
Make this change to your Gold Image and then re-provision your pools.
Make sure your SCSI Type is VMware Paravirtual.
Quick fix would be:
* Add 4 scsi controllers on the base image. take a new snapshot
* Publish to pool.
* This should avoid the issue you are facing for hot-add of scsi controller. So vCenter can use the existing scsi controllers when request received from app volumes
The suggestion i have shared only applies to LSI SAS not for paravirtual.
Second/alternate solution: You could try adding devices.hotplug=true parameter on base image advanced settings option.
Procedure to add this parameter is in this kb :https://kb.vmware.com/s/article/1012225
You just need to add 1 or 2 more SCSI Controllers to fix your AppVol mounting issue.
But I would suggest you look at getting to SCSI paravirtual to better your performance.