On Pages 49 and 50 is all of the information on the memory modules. The is no mention of the maximum amount of memory that is compatible with the motherboard. However, I believe that 16 GB (2x8 GB) would be a good amount. I suggest using CPU-Z to find the manufacturer and timings for the current 8 GB: https://download.cnet.com/CPU-Z/3000-2086_4-10050423.html
This information is in the Tab titled "SPD". You will have to remove the 2x4 GB of memory and replace it with the 2x8 GB. Once you know the manufacturer of the current memory you can buy the same memory. It can be DDR4-3200 MHz (1.2v).
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