It should be fine. Vic is correct that a standard map you could easily fit in 6-8 cities. Not all of those cities would have luxuries and such, but that is besides the point; there was enough room to get a solid 6-8 cities. If Civ 6 is ~20% larger, that should ease up the greater demand on tile improvement mechanics. As for what "ideal number" is good, I don't much care. All I care about is that it is always beneficial enough to settle cities so the map gets filled up. Having huge swaths of land go unsettled was never much fun.