The number inside the unit symbol is the strength of the unit in terms of the best combat skill of the most common element (fighter, archer, undead, giant, etc). So an infantry unit with 16 written on it will have 100 men (or demons or unleaded or whatever) most of which will have a 16 weapon skill. Within each unit there will also be some boss types. Roughly speaking, for every 25 men in a unit there will be 1 man with a +5 skill and for every 100 men there will be someone with +10 (or more). Note, the symbols next to units that don't appear in the legend are special NPCs.