There are three limitations on the amount of RAM that you can put into most Macs:
1. The number of slots in the machine.
2. The size of memory available on one RAM module.
3. Limits to the memory addressing of the memory controller or OS.
Your Mac has 2 slots, so it can only add 2 of whatever size memory cards are available. Very often, when Apple releases a Mac model, only certain sizes are available, with larger memory modules being manufactured at a later time. Apple usually doesn't make a statement on compatibility of newer modules at a later date.
Some Macs are limited to only address a certain amount of memory, either by hardware limitations, or by the OS. Adding more may not give any benefit (with only the machine's maximum being addressed), and in some cases adding more can cause poor performance. (For instance, the 2009 MacBook could address up to 6Gb of RAM.)
Your Mac can cope with up to 16 GB of RAM, if running Lion or higher.