You probably have one of the newer i7 systems that uses the cheaper 1156 pin cpu and only supports dual channel memory (as opposed to triple channel for the higher end i7 systems) - hence you are probably maxed out at 8Gb - or 4 memory slots x 2Gb each. 

They can probably sell you 4Gb memory modules, but they are very expensive and are used for servers right now - I can't even find them at online vendors.  Without knowing your motherboard, your chipset probably maxes out at 16Gb memory.