Is Paris densely populated

It depends on your point of view. It is the most dense area in France. How dense it is is about this:

If you went to paris drew a rectangle that is 1 metre by 50 meters long, there would be around 1 person inside it.

If you're in the U.S. and you're not familiar with metres, then if you drew a rectangle 1 yard by 60 yards, there would be a person inside it.