In September 1995, John Blackburn asked a simple question: "Where does everyone live (in Middle-earth)?" I wrote an off-the-cuff
essay on where people live in Middle-earth in response. Trying to list all those peoples and their locations got me to thinking about something, and that's "How many people were there in Middle-Earth?"
The question has always tantalized me because many people incorrectly equate Middle-Earth society with Medieval European society, even though there are many cues in LOTR which indicate Tolkien was not basing his societies on Medieval Europe.
For instance, the armies are too large to be supported by Medieval economies. Theoden told the messenger from Gondor he had hoped to lead 12,000 warriors to Gondor. Few medieval armies actually were that large. Even the crusaders moved around in smaller groups and came together only briefly because they couldn't solve the logistics of supplying large armies.
Fewer than 3,000 men marched to Minas Tirith to support the city against Sauron's assault under the Morgul-Lord, and yet they expected about ten times as many reinforcements. More than 30,000 troops attacked Minas Tirith, in fact. Also, when Aragorn led the Army of the West against Mordor, he started out with about 7,000 men, ended up with barely more than 6,000, and had to face off against 50,000+ Easterlings and Orcs.
Tolkien does indicate a strong Germanic influence for his Northmen, though, and perhaps he had in mind some similarities between the Northern Peoples and ancient Germans beyond language.
For instance, historians have a difficult time estimating the numbers of the Gothic peoples because they were constantly splitting up and reuniting. The largest group, however, is estimated to have been no more than 100,000 people with 15,000 warriors. These Goths lived in the 5th Century and were supported to a large extent by the huge estates of the Roman Empire.
Still, if we equate the Rohirrim with the Goths in terms of numbers, there may have been been 80,000-90,000 Rohirrim. The army which attacked Helm's Deep was estimated (I believe) to be about 10,000 warriors consisting of Orcs, Half-Orcs, and Dunlendings. If we guess the Dunlendings made up half the force, that means they were fielding an army of about 5,000 men. Maybe another 1,000 would have stayed in Dunland. That leaves us to infer about 45,000 Dunlendings (and to assume they would have had equal economies).
How many people were there in other lands? I can't really guess at Gondor's population with any hope of accuracy, but if Gondor could field 30,000 men at the end of the Third Age, there may have been as many as 100,000-150,000 more who were unable to take up arms for lack of training (in "civilized" societies, fewer men have martial skills than in "barbaric" societies). Hence, Gondor may have had anywhere from 750,000 to 1,000,000 people.
What about Esgaorth and Dale? They probably had much smaller populations than Rohan, so figure maybe 10,000 people apiece (okay, 20,000 apiece). Thranduil led only a few thousand warriors to Dale in the Battle of Five Armies. If Celeborn and Galadriel had a similar number of Elves under their command, we can guess there may have been about 20,000 Sylvan Elf warriors in the two kingdoms. But how many Elves does that represent? Probably no more than 45,000 - 50,000, given the long life-spans of Elves.
Dain led 500 Dwarves to Dale in the Battle of Five Armies. Tolkien suggests that Dwarf-women made up about a third of the population. Given the perils that came from the east, Dain would have been unlikely to leave the Iron Hills undefended, so maybe another 500 armed Dwarves stayed home? Pure speculation, and yet that suggests about 1,000 male Dwarves fit for combat in the Iron Hills in the 30th Century. Thorin said he had relatives further away than the Iron Hills, so figure the House of Durin could field several thousand Dwarven warriors -- say, 5,000. Pure speculation. Nonetheless, from that we can infer a total population of less than 10,000 (Dwarves are long-lived too).
There were fewer than 400 Mannish houses in the four villages of Bree-land, so we can infer there were fewer than 3,000 Mannish Bree-landers. I don't recall any numbers for Bree-land Hobbits.
Halbarad brought 30 Dunedain from the north to help Aragorn, but his words that that was all he could gather of their people in such haste implies there were more than that (and several Dunedain had recently been slain by the Nazghul when they entered the Shire). So we have nothing to work with in guessing at the numbers of the Dunedain of the North, but since they were unwilling to dwell in an open kingdom, perhaps there were no more than a couple of thousand.
The population of the Shire was probably easily over 100,000 -- perhaps as many as 200,000. It's really hard to tell, since we saw so little of it.