Hey :-)
What do you think is the ideal RAM size for a motherboard running nvOC?
Here is a look at how much it uses one one of my rigs with 7 1070's:
m1@Miner1:~$ free
total used free shared buff/cache available
Mem: 8107204 1632364 5565128 42012 909712 6055112
As you can see, I have 8 Gb but it uses less than 2 in REMOTE (no local display). I tend to buy HW that I can re-purpose in the event that mining goes south, so I recommend running a single 8 Gb stick per mining host. I typically purchase them as a 16 Gb "kit" with 2 8 GB DIMMs. Clearly, you could go as low as 4 with no issues but I have no use for DIMMs that small outside of mining.
I think it depends on card counts and types.
As stubo said, I think 4Gb is enough too.
1x 1070 + 2x 1060 test rig:
total used free shared buff/cache available
Mem: 8135504 1061156 5751296 31272 1323052 6703400
Swap: 8388604 0 8388604
8 x 1070 :
total used free shared buff/cache available
Mem: 8130664 2414736 4060072 62760 1655856 5228228
Swap: 8388604 0 8388604
12 x 1060 :
total used free shared buff/cache available
Mem: 8130656 2281908 4463608 86440 1385140 5317732
Swap: 8388604 0 8388604
6 X 1060 :
total used free shared buff/cache available
Mem: 8135612 1571280 5353460 56224 1210872 6125424
Swap: 8388604 0 8388604