Did you know ? If you purchased a Cisco 5508 WLC with a 12 access point license you just limited yourself to 487 access points?
The Cisco 5508 is licensed based which means you can add access point licenses as your wireless grows. The Cisco 5508 allows a maximum of 500 access points. This is a new model for Cisco Wireless Lan Controllers. 
The now legacy 2000,2100,4400 and WISM1 were licensed by the hardware itself.
You can purchase Cisco 5508 WLC with a 12,25,50,100,250 or 500 access point capacity. Or you can purchase what Cisco calls adder licenses in the quantities of 25,50,100, and 250 access points after the fact.
The license limitation becomes an issue with your initial purchase of a 5508 with a 12 access point license.
Since Cisco only resells 25,50,100 and 250 access point licenses the MAX you will ever get on your WLC is 487 access points.
Note: A 5500 Series WLC with a base license of 12 can only support up to 487 total APs because only 25, 50, 100, and 250 adder licenses are supported.
Read:
Understanding Cisco 5508 Wireless LAN Controller Licensing
http://www.cisco.com/en/US/products/ps10315/products_tech_note09186a0080b78104.shtml
p.s. Thanks Patton for the link!