Late 2012 21.5" iMac/i5/2.7GHz and 2.9GHz have 2 slots that accept 2*DDR3 laptop RAM, so you can fit up to 2*8GB. I don't know about any single 16GB RAM in one module that would fit on this model yet. There is talk (that's one article) about upcoming single 16GB RAM DDR3 modules but I don't believe any is available yet.
You can verify your configuration and the amounts of available slots here to see how different models compare, and you can notice that the 27" models from the same 2012 line-up have 4 easily accessible RAM slots that accept up to 4*8GB RAM. So if you require 32GB RAM at all cost, you may consider moving to a 27" model. I did make the comparison, but cannot seem to save it in a link.
Please note the following:
- This series of iMacs requires a specific kind of RAM modules marked as 1.35V. The 2*8GB Corsaire RAM modules you are buying are marked as 1.5V. It is not enough to have them "Apple Certified." I am not knowledgeable enough to recommend the exact needed type of RAM and instead advise you to check other questions and answers about them first and determine what is needed.
- To access the RAM slots on this 21.5" iMac, you will have to remove the front panel, which is not as straightforward as replacing them on a regular laptop. See this guide to determine if you are up to it and/or have the needed tools.