I don't know if this has the most density of organ pipes per inhabitant ;-) but would nominate a place where an e-friend of mine plays 2 organs regularly.
She has substantially increased my interest in organs, as well as in advocacy for a decently livable life with our condition, Asperger's.

Louisa, Virginia, USA - 1401 inhabitants. 6 pipe organs of 2-4 manuals.
Also one of the best communities doing academic book indexing.

Organs there and in many towns and cities around are built/installed by the family of Grooms & Payne
http://groomsandpayne.com/
(Interesting photos documenting the process of building, from raw wood to installation)

This town(?) would have ~2000x the organ pipe density of Romania's capital Bucharest. Any you know with more?

I nominate the little Swiss village of Romainmôtier (465 inhabitants). There are two organs there: the organ of the former abbey church of St Martin (4 manuals, 35 stops) and the former house organ of Jehan Alain (4 manuals, 42 stops). That's a lot of manuals for such a small village.

This is an interesting question. Our city has 438,000 people and probably 20 or more pipe organs. I am not sure on smaller cities.