Model A1418 / Late 2012 / 2.7 & 2.9 GHz Core i5 or 3.1 GHz Core i7 Processor
is it possibly to use 2 16gb memory module?
Good question! Apple does not state what size modules to use just the systems limit (which may not be the true limit of the architecture).
Self employed contract worker
As long as you can return the module I would give it a try! Look at it this way you will have one spare slot.
Two spare slots? *By default, this system has 8 GB of 1600 MHz DDR3 SDRAM. Apple does not intend for one to upgrade the RAM in this system. *8 GB of RAM is standard and it could be upgraded at the time of purchase to 16 GB for an additional US$200. Although Apple does not intend for one to upgrade the RAM after purchase, and doing so is difficult, it is possible. This system has two SO-DIMM slots (204-pin). Unfortunately, accessing these slots requires one to pry off the display that is glued to the case and remove the logic board.
My bad - I was rushing to get to a meeting, sorry - The 21.5 model only has two slots. Apple memory guide states it can be configured with 16GB. And Mayer is correct its a bear to upgrade as you have to completely remove the logic board to get to the RAM slots.
Thanks Mayer for catching my error!
Kingston's website will show you how much RAM and what kind to use in your iMac.
It shows that the Max Memory is 16GB. But it also shows that you are unable to upgrade it because of the display.
Yes its a big job as the display & logic board need to be removed. This is not a task for a first timer. But if you do have the skills & tools its doable (review the IFIXIT guides).
As far as I´m aware there are no 204 pin 16gb RAM cards manufactured so 16gb will be the maximum anyway.