We've just implemented IPAM within the last month, so there might be an easier way to do what you're looking to do than the following. However, I think this might get you the information you need.
If you use the Subnet Allocation Wizard, you can define the supernet address as your /16 and set the desired subnet size to /24. The wizard will parse out all of the /24s that can be created within the /16 space once you click the Refresh button. Select all the created /24s and click next. If you leave the defaults on the next screen of the wizard and click the Done button, the wizard will create the /16 supernet and add all of the selected /24s to the supernet. IPAM will then begin to scan all of the subnets within your supernet. Once you have added and scanned all eleven class B networks, you can go to the Reports menu on the Home tab and find the IPAM: Current Status reports. There is a report called IPAM - All Subnets that will display statistics for each subnet in IPAM. It will show you the IP % usage as well as the number of IPs used by subnet.
Although this takes a few steps, it should get you what you need.
Hope this helps!