16 or 32 gigs of RAM in a 21.5 inch 2017 iMac?

If you are not in the US, OWC has a UK/European partner:

https://www.megamac.com/collections/owc

I believe Crucial also has a site to serve the EU.

I share den.thed's concern over not ordering directly. Within the last two weeks I saw a post about RAM where it was ordered from a site similar to Amazon and the RAM did work properly in a Mac. Crucial and OWC will ship you Mac-friendly RAM.

If they are going to open up the iMac it’s best to just add the max memory the machine will take and you can afford I updated my iMac to max memory which was 16 for that model and my Mac mini I went ahead and added 64 .

agree with everyone to only use own or crucial memory becuse these Macs are picky .

If there is any question that 16GB will or may not be enough in the future?

Then 32GB is the only answer.

OWC is also the way to go for the SSD replacement.

Agree with den.thed, on my 2011 iMac I was continually low on RAM when it had 16 GB, 24 GB has cut that down, will go 32 GB one of these days.

Mac's are very picky and PC modules are not the same as iMac modules. Your best bet is to buy RAM modules for your iMac, directly from either Crucial.com or OWC macsales.com.

Well, OWC is the way to go once you need RAM bad enough. :)

Personally, I wouldn't buy RAM anywhere else and I've also bought several internal SSDs for my laptops at OWC. And, their warranty is good and works.

If you had a 27” you could easily test the RAM and return it if there is a problem. Not so easy with the 21.5” because it will need to be reopened.