16Gb. You buy it in a single kit, 4x4Gb or 2x8Gb. The 4x4Gb is slightly better performance, but you have to watch how it's installed or you end up with 2x dual instead of a quad. Don't opt for 1x16Gb, that runs single, so you'll loose upto 20% performance over dual. To get 12Gb would mean mixing kits and thats a bad idea from the get-go as there are no triple channel kits in ddr4, that was basically phased out with ddr2, newer ram running 12Gb would be 2x ram in dual and a single stick in single channel, which is quite slower than all dual or quad channel ram.