Garrison for the Rammas Echor

[Inziladun]

The answer partially depends on whether the garrison in question would be expected to totally stop invaders for an extended period of time, or simply slow them down while taking toll of them. If the former, I doubt Gondor alone could have managed it without leaving the City defenseless.

Yet (Minas Tirith) was in truth falling year by year into decay; and already it lacked half the men that could have dwelt at ease there.

ROTK Minas Tirith

So it appears Minas Tirith did not have the manpower on its own.

And that was all, less than three thousands full told. No more would come.

ROTK Minas Tirith

The outlying fiefs could initially only supply that small number. Once the southern areas were freed from the threat of the Corsairs, that enabled them to come to the defense and fully man the City, but I don't think they could have done that and guarded a thirty mile long wall at the same time.

With the additional forces of the Rohirrim the Rammas could probably have been garrisoned completely, but in the end I don't see them holding the line indefinitely against Sauron.

[Kuruharan]

Even with the addition of the Rohirrim I don't think 30 miles of walls could have been fully garrisoned.

That wall wasn't a good idea to my mind...not that in the event it mattered as Gondor lost it so quickly.

[Gordis]

Rammas was not a good idea, I think so too.

Gondor's natural defence line against Mordor was the Anduin. It would have been simpler and much more efficient to guard the bridges and crossings better - to build a new fortress in the ruins of Osgiliath by the Bridge, maybe a strong fortification on the East bank as well.

[PrinceOfTheHalflings]

I think that the Rammas was partly symbolic, but also it would have been considered necessary to guard the farmlands and settlements outside of Minas Tirith itself. Had the Rammas not existed then small parties of orcs could have been constantly raiding the farms, burning crops and playing havoc with the city's food supply (as well as lowering morale) for years before the War of the Ring.

Of course there wouldn't have enough men to man all 30 miles of it - but then enemy forces couldn't have attacked the entire length of the wall all at once. You would just apply defenders to where they were needed.

I think Tolkien may also have been inspired by real life fortifications - the Great Wall of China (which is thousands of miles long), Hadrian's Wall - and in the twentieth century The Maginot Line which was supposed to protect France from invasion by Germany.