In building realistic fantasy worlds, one error that people sometimes fall into is not getting the number right. By this I am mostly referring to populations and armies.
Often the numbers involved are based more on modern times and not what could be expected for the equivalent time frame. Million man armies just did not exist, no matter what the Dark Lord may want.
Populations were small, and predominately rural, living in small villages and involved in farming.
Armies, except for rare instances, mostly numbered only in the thousands. Tolkien is one that gets this right, at lest for the good guys. Their forces only number in the thousands, even at the Battle of Pelennor Fields, Hornburg and the Black Gates. Of course the Uruk always number in much larger numbers…
I came across a couple of interesting article on the web dealing with these matters, and offering helpful advice on dealing with realistic numbers. They will certainly help me in my world building and writing.