The units are represented by three or one. How many does each unit represent? A hundred? A thousand? I assume that when I build say a warrior and three pop out, that represents three hundred or three thousand right?
A) You can set on the options screen how you want your units to display, either three or one depending on your preference.
B) If set to three, the actual number that appear on your screen represent the relative health remaining for the unit. I'm sure there is a formula, but my rough estimate is something along the lines of 90%+ is three, 50-90% is two, less than 50% is one.
As for the actual number of soldiers the graphic represents, I don't think there is any hard and fast rule either, but clearly it more than three. I tend to think of them something along the lines of a platoon or some other small but functionally effective fighting force. I also think it would vary depending on the unit type. Is a "small but functionally effective force" of knights the same quantity as a "small but functionally effective force" of pikemen.
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Also you can use mods to increase the unit number to more which is visually cool but all those animations take longer to resolve.