Q1) How to get the report on current license consumption to determine the number of licenses may need to purchase ?
--> It is on the road map and will be available soon.
Q2) How the licenses are consumed? If a site registers an AP via mac-address, does it always consume that license, or is it only active APs?
--> No, AP license get used only when they are assigned to a site. AP's which are just learned by CMX and not assigned to any site will not consume any License.
Q3) How the licenses are consumed and if any are being wasted on decommissioned APs.
-->Make sure to remove all the decommissioned AP's from site and update it with new AP's learned by CMX or add them manually.