Surely this is just a matter of preference: the forum’s responsiveness will be down during an upgrade, but it will still be available. The upgrade will take a little longer with fewer cores.
I suspect the OP is focused on maintaining performance 
I’m using deprecated CX32 by Hetzner for 2-container plus mail-receiver and it is 4 vram 8GB for really low activity, but I have 1000-1500 lurkers human visitors a day. I needed 4GB swap for rebuilding, though — I don’t know if that need raised from Discourse itself, not because of 2-container 
I know this isn’t telling what is needed, but:
root@foorumi-hel:/var/discourse# free -h
total used free shared buff/cache available
Mem: 7.6Gi 4.2Gi 787Mi 542Mi 3.4Gi 3.3Gi
Swap: 4.0Gi 715Mi 3.3Gi