The first iMac 5K supports 32 GB max. The iMac 5K late 2015 supports up to 64 GB .
http://eshop.macsales.com/shop/memory/iMac/Retina-5K2015/DDR3L
Would you need that much RAM? Most users not. Music & video editors do, graphics design professionals do. When I make a 400+ pages book in InDesign with over 900 images 16 GB of RAM just isn't enough. It's impossible. I tried, either the OS or InDesign crashes. 32 GB is great. Also with 32 GB of RAM most of the time there is no need for memory compression or need to swap which improves performance greatly, especially on older systems.
I use a late 2009 iMac 27" i7 with 32 GB of RAM which I will retire in the next year.