I was giving Elleth's idea some thought.
To only be able to gather thirty men for a mission of that importance implies it seems either an extremely sparse population, or very heavy commitments elsewhere
I realized that I was thinking at the problem in a myopic and self-oriented way. And that my first instinct was to view the situation from my own perspective. By way of explanation, I have a social media account that I don't often view, that I largely use to keep in touch with my acquaintances around the world (mostly in the US of course). A few hundred of those acquaintances are brothers-in-arms. Not long after I got out in 2015 (less than a year later, in fact), a brother that I went to war with contacted me about going to Turkey and joining up with a YPG group to fight ISIS. I declined, but instead offered to help him in other material ways. To his credit, he followed the prompting of his conscience and went anyway, spending just under a year as a volunteer. So, without going into longer story, that was in my head thinking about Elleth's question. In my mind, Rangers are elite units, far better trained than a local levy (even of hard people in hard times)...so I was thinking along the lines of getting anyone (much less a full platoon's worth of warriors) to drop all commitments at a moments notice and ride out is kind of a miracle.
However, I realized that's not Middle Earth. (duh smack! facepalm)
After that moment of revelation, I thought that I had better check out Tolkien's references to calls for warriors and the like.
Right after the council of Elrond in TFOTR at the beginning of "The Ring Goes South", Gandalf tells the hobbits "...Some of the scouts have gone out already. More will go tomorrow. Elrond is sending Elves, and they will get in touch with the Rangers, and maybe with Thranduil's folk in Mirkwood. And Aragorn has gone with Elrond's sons..." I had forgotten all about this, and it's kind of important to this idea, because it essentially gave the Rangers a warning that great danger and dark times were at hand, and to be ready. So, +1 for Elleth's idea that the population was really low or that Rangers were already heavily committed.
WARNING: Self Congratulation Here!!! Please avoid if Grimolt's smugness may irritate you! After Moria, when the fellowship reached the outskirts of Lothlórien, they came under the aegis of Haldir and a few of his fellow border patrol members. A "strong company" of orcs (a hundred he states later) ostensibly hunting for the fellowship, had invaded the boundaries of Lórien, and Gollum used the confusion to try and get close to the ring. Haldir interrupted Gollum's attempt, causing Gollum to flee, after which Haldir explained to Frodo that he couldn't fire at Gollum and raise a cry, because they couldn't risk battle. He goes on to state that he sent a messenger (Orophin) back to his people to warn them of the orcs, and that the remaining three were luring the orcs further into the wood with the result that "...None of the orcs will ever return out of Lórien. And there will be many Elves hidden on the northern border before another night falls." Nice validation by the old man of how to use scouts here.
Ok, anyway, the pertinent levies I found were in Rohan. Rohan already had a sort of standing military force in the Riddermark that seems reminiscent of Anglo-Saxon lords and their Húscarls. After Rohan realized the true threat of Orthanc, and again when the host rode off to the battle of Pelennor fields, there was a call out for the levy. Before the battle of Helms Deep, Théoden called out the levy saying
"...The host rides today. Send the heralds forth! Let them summon all who dwell nigh! Every man and strong lad able to bear arms , all who have horses, let them be ready in the saddle at the gate ere the second hour from noon!"
At the beginning of the chapter, Tolkien explained that Gandalf and the three hunters had arisen at dawn, and made their way to Meduseld in bright morning, so Théodens call for the levy to be ready by 2pm indicates an immediate hasty call up of every military capable male within a few mile radius of Edoras. Timeframe was something like "we are going to eat lunch right now, and then ride out", so the call can't have gone more than 20 or 30 minutes worth of travel away.
Tolkien doesn't give us much detail on the numbers of Théodens host, but when they were consolidating with the force Erkenbrand left at Helms Deep to protect the people of the Westfold that had gathered there, Gamling stated he had about a thousand men fit to fight on foot, and that three fourths of the population of the Westfold (women, children, and the old) were gathered in the caves.
We still don't gain the benefit of solid numbers, but context sure makes it sound like at Helms Deep Théoden's total fighting force was a thousand or so that he brought, plus the thousand of Erkenbrand's rear guard.
On the last day of battle, when Erkenbrand and Gandalf showed up, Erkenbrand had a thousand dismounted men that charged into the orc flank, driving them into the trees (of doom) that had shown up mysteriously (at Treebeards command, we find out later). All told, a total hasty mobilization of Rohan's core city and the Westfold resulted in something like 3-4k troops.
Later on, in ROTK "The Muster of Rohan", after Théoden called for a general mobilization of the whole country, when Théoden received the red arrow from the messenger of Gondor, he expressed his regret that his people were so scattered, and remarked,
"Ten thousand spears I might have sent riding over the plain to the dismay of your foes. It will be less now, I fear; for I will not leave my strongholds all unguarded. Yet, six thousands at the least shall ride behind me."
So, total mobilization of all Rohan would result in a force of approximately 10k, of which 3/5ths could be used as an expeditionary force. Using loose stats that indicates that the total population of Rohan was around 40-50k. Of the 6k that Théoden could take on expedition, even though they were his best 6k, I'll make a wild guess that only about 10% of them were high quality troops...essentially the household troops of Théoden, Eomer, and Elfhelm if we go by the identified Marshals of the Mark. So, (again, sheer speculations here), with that in mind, I'll speculate that each household had an elite fighting force of about 200, more or less.
If we look at the Grey Company as Aragorn's household troop keeping in mind that he calls them kin, (but then again, all Dúnedain are kin), and then apply the same 10% ratio to a complete mobilization, that would give us about 300 total military age males, resulting in a population of Dúnedain in the North of around 1200 or so. That wouldn't count the common folk who lived in the area that used to be Arnor, like the Breelanders and such. It's all speculation, of course, but a community of Dúnedain that small also explains why their leader is called a Chieftain, (not a king in exile), and why the heir is always fostered in Rivendell.
So, long story short, even though my numbers are unscientific and only loosely supported by the texts, I'm convinced now that Elleth was right on the money in identifying the Dúnedain of the North as being extremely scarce in population, and likely already committed to internal rangering tasks. Really, the only people free to go support Aragorn would have been his closest friends and household, and that would be the Grey Company.
Thank you again friends, for the joy of scouring the books and thinking through these questions.