The 27 inch iMac for 2019 (5K, 2019) introduced DDR4 memory at the 2666 MHz speed, and an upper limit of 128 GB RAM. DDR4-2666 (PC4-21300) non ECC SODIMMs. Apple originally stated a 64 GB limit, however 32 GB modules have been tested and work in a 128 GB configuration.
(As usual, the 21.5 inch (4K, 2019) machine is not user-upgradeable in RAM, although a professional installed can strip the machine down to access the needlessly buried sockets.)
The 27 inch iMac remains at four memory sockets, which should be installed in pairs of memory. The standard configuration is 2 x 4 GB SODIMMs for 8 GB total (which today is the absolute minimum that a machine needs to run, 16 GB is the practical starting point for running applications)
You can install 2 pairs with different capacities, so you could add 2 x 8 GB to the stock 2 x 4 GB for 24 GB of RAM. Later you could take out the 2 x 4 GB and add 2 x 16 GB for 48 GB total