The 27" iMac has 4 slots and it is best to install modules in pairs.
(It will come with 2 4GB modules in 2 of the slots and 2 empty slots)
The link that you have provided, is only a single 16GB module and it is not Mac compatible.
If you want to add 16 GB, then you should add 2 8GB modules.
If you want to add 32 GB, then you should add 2 16GB modules.
Personally I will only buy directly from Crucial or OWC to ensure that I'm getting the correct Mac compatible modules with life time warranty.
Beside the price difference is minimal compared to having to returning bad or incorrect modules with Amazon.