Probably the most important factor in this is how large is the Project File you have opened. So these numbes can vary greatly.
My Archicad usually uses around 100-200 MB of memory, but a lot more if it is generating a large 3D Window model or rendering. Then it can use up all the memory of the system (I have 1 GB).
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AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28