I have a doubt regarding license for monitoring LAP,
do I have to request one device license per LAP, or just consider the device license of the WLC to which each LAP is associated?
Depends. On the older wireless LAN controller models like the 2106 and the 4400s, there was no licensing on the controller.
Newer models require licensing per AP that will be joining the controller.
On top of that, regardless of model of controller or AP, each AP, switch or router in Prime Infrastructure that's going to be managed has to have a license count. (Controllers do not consume a device count themselves.)
Example: a 5508 controller that has 150 APs joined to it, and there are 10 switches and 2 routers in the network that are, as well being managed by Prime Infrastructure would require at least 162 license counts.