It depends on the map. In the Civ 6 Europe map, the area where the Normandy landings took place was 3 tiles. You could have three units attacking in the initial

It depends on the map. In the Civ 6 Europe map, the area where the Normandy landings took place was 3 tiles. You could have three units attacking in the initial wave against 1 defending unit, with various ranged units on both sides. At that point in the game, the units could be corps or armies, which would make the overall scale of fighting about right.