you’re combining a few terms here in ways that are somewhat confusing. to clarify:
a “unit” is a single component of a fighting force. “units” are what you build froma city. if you go to an elf city and double click on the “elf longbowman” icon, in 1 turn a single “unit” of elf longbows will appear in the city.
a “stack” is a collection of “units” that act as a singular, compressed force on the strategic map. you can have 6 units to a stack, and a stack takes up exactly one hex on the strategic map.
on the tactical map, each “unit” will be visually represented by a soldier, or in many cases, a collection of soldiers. if you see multiple graphical models that are all present on the same hex in tactical combat, that is a single “unit.” the number of soldiers in a “unit” is purely a graphical effect: it has absolutely no effect on fighting strength or anything else. there is no consideration to be made based on how many soldiers are on a single hex in tactical combat: all that matters is how much hp that unit has. as the unit gains and lsoes hp, this is visually represented by a soldier count, but that visual representation does not correspond to anything at all other than simply # of blows until that unit dies. a unit with 1hp can punch just as hard as a unit with 100hp.