iMac 27" Late 2015 - what ram to buy?
You will need two "16GB Kits" (four 8GB modules in total) to get the full 32GB.  I have personally done this in my 2012 iMac 27"



The memory that I recommend is Crucial 16GB Kit (8GBx2) DDR3L - PN# CT2K8G3S186DM

I recommend Crucial because they are very reliable and they have a lifetime warranty.  There are brands that are cheaper, but (IMO) it's simply not worth the cost savings.  I have found that not only is the quality better (fewer failures with lifetime warranted memory versus non-lifetime warranted)  the customer service has always been much "stronger" in terms of getting faulty memory replaced.