Andy's answer is 100% correct and I can confirm it.

Hello Mr. Baumgartner.every IMac has 4Ram slots.this model comes with 2x4GB and therefore has 2 free slots.this model can be upgraded to 64GB RAM. I recommend you to buy extra RAM directly from digitec as it is much cheaper than buying from Apple.Also I recommend you to upgrade at least to 32GB RAM to have enough computing power.I edit photos&videos on this device and have 40GB RAM installed at the moment...is still a bit too slow for me personally but is enough for professional work. MfG Andy