Both Far Cry 5 and New Dawn will (eventually) consume my RX 6800's entire 16GB buffer, at 4K/ultra/HD textures. That said, they ran fine with just 11GB, per my experience with 1080 Ti, but I had 32GB of system RAM installed; when not enough VRAM is available to be "allocated," Windows attempts to use system RAM in its place (which is quite dumb as system RAM is far too slow to be used as VRAM). I can't speak for 16GB system RAM users, but given that even with the 11GB VRAM buffer 4-5GB of system RAM were also allocated...

Long story short: if FC6 requires no more VRAM than 5/ND, 11/12GB should suffice; if it requires more than these older games--which is my guess--it probably won't. (I'd forget about 8GB; when I ran these two games with 2080 Super and 5700XT, sooner or later, the "VRAM wall" would be hit, they'd drop to 2-3 fps, and the game would have to be restarted; again, that was with 32GB of system RAM installed.)