Both the 2018 & 2020 Mac Mini’s use the same 260-pin PC4-21300 (2666 MHz) DDR4 SO-DIMM’s (x2)
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Before you go to the expense of pushing this system to 64 GB you maybe unable to leverage all of this RAM. Video & music production is about the only apps that can, But to be honest the artist I setup systems for often find 32 GB is really the ideal and getting a large internal storage 1 or 2 TB being a better setup (balance). Most have been going with iMac Pro’s as the Mac mini’s GPU is quite limited. The two 2018’s that I setup used external GPU units as they where holding out for the Mac Pro’s but needed something in the mean time.