Great question, Tom!
First off, as in Basic D&D and AD&D, most men-at-arms are 0-level normal men. With that in mind, imagine a typical feudal army. The “army” is itself a pyramid where a manorial lord has a few knights, who in turn have some sergeants-at-arms, who in turn have some men-at-arms. A 100-man warband might include 80 0-level men (men-at-arms), 8 1st level fighters (sergeants-at-arms) 3 2nd level fighters (knights), and 1 3rd level fighter (the local manorial lord). 5 of these warbands might work for a 5th level baron.
If you assume mages and clerics are distributed along similarly feudal lines, then you would assume that the 5th level baron, with about 500 men (5 3rd level fighters’ warbands) probably has a 3rd or 4th level mage as his court wizard and a 3rd or 4th level cleric as his chaplain. When he goes to war against the neighboring baron, those are the forces they are pitting against each other.
So that’s the baseline for envisioning the average conflict.
Now, if we imagine a period of Total War in a society, in which all of the classed characters could be recruited into the war – of, say, 6MM people, there could be (1 in 12) about 500,000 classed characters which form an elite army. Of these, 1 in 50, or 10,000 will be 5th level or above, and 10% of those will be mages, so you’ll have 1,000 mages capable of hurling fireballs. So that’s 1,000/500,000 or 1 in 500, or 10x the frequency that would appear in everyday life.
But it’s hard to imagine a period of total war in which ALL of the classed characters are recruited, that doesn’t have some sort of vast Levee En Masse that recruits a huge number of un-leveled characters - as cannon fodder to absorb those fireballs, if nothing else! 0 level characters are valuable because they are only minimally worse than 1st level, while being much more widely available. Moreover, some of the leveled characters will have “aged out” of adventuring, being retired, wounded, etc. Their level doesn’t necessarily speak to their current fitness for the rigors of army life.
That said, there can and will of course be areas which are “thick” with higher level characters. Such places, units, countries, regions, etc., will be considered HIGHLY dangerous and threatening. A small barony run by a 14th level mage would be like North Korea with Hydrogen Bombs… small, but terrifying.