What about Lightsabers and Jedi Philosophy makes them so rare and special?

(In Universe Answers Only)

If you notice in most of the Star Wars galaxy most Jedi carry only ONE lightsaber while some carry two. My question is really nitpicky but it makes me wonder: If lightsaber are so rare why do some Jedi carry two while others only carry 1? Wouldn't it be prudent to have "spare" on your belt?