RAM requirement for the newest iMac

https://www.apple.com/shop/buy-mac/imac

21.5" "8GB 2666MHz DDR4 memory, configurable to 32GB"

27" "8GB 2666MHz DDR4 memory, configurable up to 64GB"

https://eshop.macsales.com/shop/memory/imac-2019-27-inch

FWIW, get any RAM for a Mac from OWC or Crucial. These vendors

RAMs will be the most reliable to working in an iMac.

DO NOT get any basic generic PC RAM as they will only tend to

cause issues, especially those advertised as "bargain" or "value" RAM.

If it doesn't state that it is Mac specific, not just "compatible", it will

likely cause issues.

Also, consider the 27" models in the past, and I assume this current model, has a user-accessible door to get to the RAM slots. The 21" model RAM slots are internal and require a near total breakdown of the machine to get to, which may invalidate the warranty if done by the user, those RAM sticks would have to be built-to-order at time of purchase. (some prior models had the RAM soldered directly to the motherboard and were not removable sticks at all.) OWC tends to do thorough testing of their products and take downs to determine what user upgrades are appropriate, so check their blog for updates on info on the new models as it becomes available. For example:

https://blog.macsales.com/47657-apple-releases-powerful-new-imacs-owc-to-provide-memory-upgrades-up-to-128gb

iMac memory specifications - Apple Support

DDR4-2666 SO-DIMM 260 Pin PC4-21300. Something like that.

FWIW, the new 2019 iMacs are not listed in the link.