Chosen Solution
How does the memory bus look like in the 21,5” iMac? It has got two slots. Can I upgrade those individually or do they have to be in pairs. My guestimate of course is, that they have to pair up, but what are the facts? Reasoning behind the question: one 16Gb stick is same price as two 8Gb and would add up to 20Gb when one 4Gb stick would stay in the mac. Also at a later point I could upgrade it to 32Gb, by buying one additional 16Gb.
Yes your machine will take one 8 and one 16 GB stick. However, after you look at the difficulty involved in installation (you have to pull the logic board), you may want to wait until you can afford to do the full upgrade 32GB at one time. iMac Intel 21.5" Retina 4K Display (2017) RAM Replacement
One stick would work fine. I would only do so if you plan adding another one later as dual channel memory allows faster and better performance as the loads are split, rather than one stick doing all the work.
