Which openings require low amount of theory and high amount of theory?

Currently I know that the London System and Caro Kann requires minimal and Najdorf, Grunfeld, KID, and French require a lot. 

A lot of times, seeing the word "system" in the name (or as a common description) is a clue that an opening does not require so much theory.  In such openings, one side (usually white) plays pretty similar moves almost no matter what the opponent does.  There's the London System, which you mentioned, and also the Colle System, King's Indian Attack, and Stonewall Attack as a few that come to mind.  My friend plays the Scandinavian as his primary opening against 1 e4, since it has fewer variations to memorize.

I just thought of a few "systems" for black, in the Hippopotamus and the Hedgehog.  I also found this old thread on these forums: https://www.chess.com/forum/view/chess-openings/what-are-the-main-opening-quotsystemsquot

there are no openings that don't have a ton of theory... some openings are just more plan based and making a mistake on move 20 wont make you lose instantly... but even this does not apply at the low level... go play najdorf or grunfeld... if you understand the plan better you will win... knowing this insane theory doesnt apply to anyone rated below 2000

but it does... because it will transpose into something else eventually