Please tell us the exact machine you are referencing. OR You can look it up yourself here:

https://everymac.com/systems/apple/imac/...

UPDATE

RAM Type:PC4-17000 DDR4*Min. RAM Speed:2133 MHz

Details:*Apple reports that this model ships standard with 8 GB of "2133 MHz DDR4 onboard memory." Thankfully, however, this is not entirely accurate.

Standard RAM:8 GBMaximum RAM:16 GB*

Details:*8 GB of RAM is standard and reported as onboard with an optional upgrade to 16 GB of RAM at the time of purchase for an extra US$200.

In a pleasant surprise, as first discovered by site sponsor OWC, this memory actually is not onboard and can be replaced or upgraded after the initial purchase of the Mac, it's just unnecessarily complicated.