The sentense means: Which room does Mr Jones live in? so 一间 means a room 一间是“一间房间”的缩写，是量词。汉语里，不同的名词对应不同的量词，比如a book =一本书，a umbrella= 一把伞，a computer=一台电脑。

It's not different between countable nouns and uncountable nouns in Chinese. So every noun need another word before it to definite the nature just like an uncountable noun in English. For example .Not only "a piece of paper" （一张纸）need a "piece" before "paper", but "a room" need a word "间” before it.

间 = the shape of the room door

琼斯先生住在哪一间 means 琼斯先生住在哪一间房间. The last word "房间" is unnecessary so it is skipped.