The overlay you get by pressing the "7" key or choosing the next to last overlay icon on the right is very useful.
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If you chose a stack of units when this overlay is up, you get a menu with a lot of info that includes how many of that stack's elements can participate in combat in a target region.
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The amounts change for a given region depending on who is in command, which posture the stack is set and some other factors. I got one stack that demonstrated the ability to attack with the most elements, 51 line elements and 15 support elements (artillery). This was interesting because it was much lower than were allowed in AACW. Anyway, if you ever wonder why your whole army was stopped by less than one enemy Division in a mountain region, this is the overlay to check.