Comment by magicalhippo

TrueNAS is an OS with management bells and whistles. I'd say yeah you'd want 16GB for TrueNAS to work well, ie roughly 8GB for TrueNAS and roughly 8GB for ZFS cache.

No you do not need 16GB simply for a 12TB ZFS array on a plain Linux/FreeBSD box. It'll be faster, but you don't need it.