What era is your story set in? The trend for England in the late middle ages was for diminishing sizes of armies compared to some campaigns in the 13th century. As armies became more professional, armies got a little smaller. A rough ballpark for an English field army in France in the Hundred Years War might be somewhere between 7000-15000 men. That's for major expeditions led by a royal figure, though. Small border conflicts or castle garrisons could involve anywhere from a few hundred to several thousand soldiers. Typically, for western Europe in the middle ages, armies were not very large. The biggest forces, to my knowledge, would have been crusades involving multiple nations. Larger armies started to appear in the mid-15th century along with the beginnings of more robust state structures that were better equipped to pay for and maintain large bodies of troops.

I imagine you might be most interested in European medieval armies which unfortunately I can't answer, but in case you have any interest in medieval Islamic armies the standard Islamic field army in the conquest period seems to have been about 30,000 men. That's the number that pretty consistently pops up for the major battles of the 7th-8th century like Qadisiyah, Yarmouk and Tours.

The estimates for some of the nomadic conquerors are all over the place, but then once those dynasties become settled and return to slave-soldier armies we return roughly to that 30,000ish number as at Manzikert in the 11th century.

Some of the interesting debates are over what the possible size of certain nomadic armies could have been, particularly for the mongols. The estimates are all over the place with the low end limited by modern historical estimates for just how much forage you would have needed for a horse-borne army with strings of 3+ horses for each soldier in reserve.

edit: word