Some of what is below comes from me, but most is a summation of an article I found here:
http://www.suite101.com/article.cfm/tolkien/64660/1
I'll try to summarize it, but feel free to read the entire article.
I think one of the thing that gives insight to the Rangers is the comment that they were defending the Shire and Bree from many evil things. We know at the Ford of Sarn that they tried to defend it from the Ringwraiths, though they had to give way after suffering loses (Unfinished Tales). Later we know they are no longer defending the Shire or Bree because both suffer an intake of Saruman's ruffins, though Frodo at Amon Hem lets us know they were probably fighting orcs that were coming into Eriador (and thus may explain why Halbarad could only raise 30 in a short time to go south, not because there weren't more. We do know that Tolkien said they lived in the wilds east of Bree, indeed at the Angle due to a note now at Marquette University.
However, Rangers themselves may have been specific officers of the Lord that patroled the former kingdom of Arnor, now Eriador. Thus Aragorn's people were probably not wandering the wilds, they probably resided at the Angle and perhaps a few other locations, and numbered in the thousands (not 100's but like 3 to 5 thousand). Certainly the Rangers were a professional force of military men who patrolled near their home, and through Eriador to protect themselves and the inhabitants of Eriador.
So if Dunedain of the North lived at the Angle and a few other small settlements, who wandered? The Rangers being representatives of their Lord or Chief, were the wanderers. Why do this with Rangers? Probably because though they could no longer deal out justice and law, they could support local law and custom. Furthermore by maintaining a corp of Rangers, the Chieftons maintained their claim to the Lordship of Eriador, and thus their claim to royality. Why was this important? Because it was for told that one of their number would someday rise to great height, and that had to give them hope in the restoration of their kingdom.
This notion of the Chiefton claiming royal authority in the north is important with Aragorn. His titles as proclaimed by others always listed him first (or by himself as in front of Minis Tirith after the Battle of Pelannor Fields) is Chieifton of the Rangers of the North, and Captain of the West. This idea of Chiefton then shows that he has royal authority and has led a military force, and Captain of the West shows he has now led Gondor's forces in victory (as the last King of Gondor who had been given the crown had also came to claim the throne after winning a miliatry victory. This set the precedent for Aragorn. So the title of Chiefton was important as was their role patroling or wandering over Eriador since it maintained the claim of royal authority by the Cheifton who led them.
So how many rangers were there? We could estimate that at Sarn Ford there were probably around 30 (since Boromir makes it clear that a company of his men could not hold the Black Riders back at Osgiliath). We could guess that this company may have been has large as 50 and given the task of patrolling the southern border of the Shire. From the Weathertop chapter we know that Rangers also operated in small groups as witnessed with the left firewood in the dell at Weathertop. These men may have been patrolling from the angle to the ruins at Deadmen's Dike to ensure protection of their communities. We would also have to assume that Rangers protected the road from the west to Rivendell and kept it open.
I am glad that I found this thread and the article that I have tried to summarize here. It really made me think. I don't think Eriador was full of tons of people, but I also don't think it was as empty as we make it. Not only did we have the Dunedain, but you had shrinking numbers of Elves, but Elves still wandered here and were present. Dwarves surely existed in Eriador due to the road and the Blue Mountains. We know there was Bree and The Shire, and probably other settlements of men. I have always been fascinated by the Rangers of the North since my first reading of the books some 31 or so years ago. I look forward to learning more!