You can order any number of license as per your requirement as mentioned below.
Also the installation is easy using Controller Gui and under license tab you have to select/apply the license file which you receive.
Example:- If you have X no. of AP licenses already and your requirement is Y in total AP licenses then Z=Y-X is the total license you have to order.
Check the link below for further detail.
Ref: http://www.cisco.com/c/en/us/products/collateral/wireless/5500-series-wireless-controllers/data_sheet_c78-521631.html