4 RAM Slots
RAM Type: PC3-10600 DDR3 Min. RAM Speed: 1333 MHz
Details: Supports 1333 MHz PC3-10600 DDR3 SO-DIMMs.
Standard RAM: 4 GB Maximum RAM: 16 GB
Details: By default, 4 GB of RAM is installed as two 2 GB SO-DIMM modules. Two slots free.
To start, I would just buy two 4 GB chips and upgrade to a total of 12 GB.
This iMac supports a maximum of 16GB of 1333MHz DDR3 SDRAM. Depending on your budget you can for the (4) 4GB sticks at now or buy (2) 4GB sticks now and a additional 2 later.
note: this speed of memory is every common right now so DON'T trow away the (2) 2GB sticks, you may need them one day.