Which has more theory to Learn for Black: Classical e5 or Sicilian?

It's not either you understand it or you don't, it's a continuum. No one has mastered either opening in the sense there is nothing left to learn or no way to improve their play. Everyone is learning.

You should play in OTB tournaments. You'll find out how useful knowing theory is... I don't think it's as important as you think it is.

All the time you'll enter some line and on move 10 (or whenever) you'll no longer know what to do. Within the next 10 moves it will turn into a knife fight (so to speak) and the winner wont depend on who knew more theory at all.

Theory only helps you win a game when:

1) Your opponent blindly follows a line you happen to have memorized and ends with you being at a clear advantage

2) You not only memorized many variations, but you understand why they work, why the bad moves don't work, and you've studied entire games so you know what to do when the theory runs out.

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But there's another thing that will happen when you play OTB. Not only will you get to move 10 and realize none of it mattered because now neither of you know what to do, but sometimes your opponent will leave book early, on purpose, with "nonsense" moves or ridiculous sidelines that aren't supposed to work. Again at some point in the middlegame there will be a critical position, you'll do your best to figure it out, and the result of the game will hinge on moments like that which have nothing to do with the opening.

So should you study openings? Of course.

Should you worry about which ones? No. Just pick something that leads to middlegames you find interesting.

And the best part is you can kill two birds with one stone... the way you figure out which opening to learn is the same way you study the opening... by playing over GM games.

Play over 1 or 2 GM games a day. Quickly. DON'T try to understand every move, that's not the point. DON'T spend a long time on a game, you should be able to get through 2 games in under 30 minutes easily.

DO notice common piece placements and common pawn breaks. DO notice which side of the board (kingside, center, queenside) the players played on. Make a few notes of these things in a notebook. At first it can seem random, but as you play through more and more games you'll notice commonalities... you'll start to predict where the pieces belong, and you'll start to predict what each player is wanting to do in the middlegame.

Play what you like, and what is strong. Make studying it a joyful habit that enriches your life in and of itself. Enjoy the adventure of learning and do not fear variations.

I always loved the Kings Indian and Sicilian, but I was afraid of all the theory. I spent what may have been the best years of my chess life playing openings that were weak and fairly unappealing instead, always switching from crappy variation to crappy variation. Not recommended 😂

https://thechessworld.com/articles/training-techniques/how-many-hours-per-day-to-work-on-chess-according-to-gm-shipov/

"However, for an under 2500 rated chess player, it is a waste of energy to spend that much time on the openings. The training on this stage needs to be balanced. All stages of chess games needs to be developed equally.

I have seen 2000 rated players that understand opening theory on level of strong International Masters. They know have memorized all opening lines and variations, but when you leave the theory behind, they quickly collapse in the middlegame, and know even less about the endgame. And that is funny; just imagine an athlete, with one very strong arm, and anorexic rest of the body. A good chess player needs to be balanced. In your individual work on chess, you need to take that into account, depending on your current level.

If you are an under 2200 player you don’t need to study openings that much and deeply dig into the theory. You need to spend most of your time on endgames, tactics and other fundamental elements of chess."

Six?

Perhaps something similar is somewhat true for the Spanish, Italian, etc.

"Building a repertoire … The first step is to think about your personal style. Do you prefer open, tactical positions or closed, strategic positions? Does an attack on your king make you nervous, or are you happy so long as you have a counter-attack? Do you prefer main lines, or something slightly offbeat? ..." - GM John Nunn (1998)

"... Each player should choose an opening that attracts him. Some players are looking for a gambit as White, others for Black gambits. Many players that are starting out (or have bad memories) want to avoid mainstream systems, others want dynamic openings, and others want calm positional pathways. It’s all about personal taste and personal need. ..." - IM Jeremy Silman (January 28, 2016)
https://www.chess.com/article/view/opening-questions-and-a-dream-mate

Might the result of the game be influenced by the degree to which one has managed to avoid difficult positions? (Things like 1 e4 e5 2 Nf3 Nc6 3 Bc4 Nf6 4 Ng5 d5 5 exd5 Nxd5, for example.) Might the result of the game be influenced by the degree of understanding that one has acquired while learning about the opening?

Did SeniorPatzer say something about an intention to memorize all opening lines and variations? Did SeniorPatzer say something about not paying much attention to middlegames and endgames?

"What is your favorite opening? ..." - CanterburyRocks (April 4, 2019)

"White: English.  Flexible and not a lot to have to commit to memory. Black: Taimanov Sicilian.  Again, not a lot of memory commitment.  Benko Gambit.  Easy to understand pawn structures." - IMBacon (April 4, 2019)

https://www.chess.com/forum/view/general/favorite-openings-2

Here we go again

Did anyone indicate otherwise?

Will you next be explaining how the rook moves?

Did you ask about anyone needing to be told that all openings involve principles. Did you ask about anyone needing to be told that endgames are important?

Well, I think we do.

Your comments can be seen above as you posted them. I see no need to reproduce everything.

I wrote "I think" because I have not asked others.

I wrote "I think" because I do not think of it as fact.

I do not think it is always necessary to quote in full. I do not have an agenda served by misunderstanding. You are, of course, free to post any argument that you have in favor of a claim that misunderstanding has resulted.

One of my reasons has been to call attention to an attitude towards openings that seems to be different from "Just follow opening principles." Also, I have tried to draw attention to whether or not there is any reason to believe that anyone here needed to be told that "all openings involve principles."