How many Rangers of the North were there ?

[shadowfax]

Point taken for the Romans. Maybe also Gondor, being a properly organised state, modelled maybe on the Roman empire, would have had a properly enrolled and salaried army. But most other countries and states of Middle-earth probably lacked that level of sophistication.

Also, I do accept that Aragorn probably didn't have any job besides being a Ranger. He was in the fortunate position of being in Elrond's favour and would have received support from Rivendell.

Maybe some other rangers besides himself also enjoyed that priviledge, but there would have been a limit to how many of them (and their families) Elrond's purse, generosity and goodwill could have supported.

The fact that the rangers were keeping watch does not mean they were on sentry duties. It could also mean that they were keeping a watchful eye while going about other things. They may have lived in scattered homesteads and would have been aware of anything that moved in their vicinity and would have taken action as required.