How much theory can you remember at a maximum

Yes, to an extent

But eventually players around 1500ish actually have to start knowing at least what to do in certain openings or you have 1800 rapids doing stuff like this

A lot of times, understanding an opening will lengthen the memorization.  For example, here's one line I know like the back of my hand that is 29 moves deep:

That is why i said "really work on openings".  Obviously before that level you should have some idea of what to do with the openings you play.  But chess players that like i said barely know what to do with the basics, but thinking openings will cure all their woes? Just wrong.

Just to illustrate...

Every year n Reno GM Sergey Kudrin does a clock simul.  You come back the next day and he goes over the game with you pointing out what went right, wrong, etc.  A kid sits down and proceeds to tell a GM how he is a "tactical/aggressive" player.    The kid had the black pieces and of course played the Sicilian.  They got to a position and the kid asks Sergey: "When I get here i don't know what to do?"  Sergey in his deadpan Russian accent says: "Well...if you don't know what to do.  Why are you playing that opening?" 

Laughter ensued.  I felt bad for the kid, but when you play with fire you get burned.

I’d say 2000 otb 

Maybe 2200 

Ive heard the saying that a person with enough skill can become master no matter how losing they are out of the opening

But I can say from experience that beyond that it definitely matters 

Wasnt it Lasker that said anyone could become a Master as long as they were willing to put in the effort?

Idk if it was lasker

Idk what's the most I remember, or which lines I remember are theory, and which ones I've found myself. It also depends on how many sidelines there are, and how long they are.

Alright, I could remember 10 moves. Beat me.

Probably accurate... in my most recent otb I got smoked with a sideline in the g3 modern benoni (Qc7 line), so I looked it up, and four rounds later... I played against the exact same line! And I won.

lol

I think a bit lower should do. I'm already losing on the opening stages and there's definitely no way to cure that

can reproduce around 20 moves in some main lines of Marshall Attack, but it's mostly the same first 10~ moves in all variations.

One thing you have to realize though, not all 20 moves of theory  are created equal. Some 15 move lines involve the potential for lots of sidelines, in which case a good player likely has at least a 3-5 move continuation for all the respectable replies outside the big main lines ,whereas some lines can be 20-25 moves deep but they are either a critical line where you just have to know them if your opponent goes full stockfish on you in narrow corridors, or are thematic in a way that , you can get 8-10 moves on just being the  only idea making sense. 

The really tricky ones to remember are sharp positions or extremely subtle positional lines where at any given moment you have to remember entire branches where playing the right move in the right order is crucial. Not only can the number of lines you need to remember can b huge when added all together, but you often cant rely on general positional motifs to figure it out if you forget.

true. It's mostly about how much lines you know rather than how deep.

In short. Nthing

Ye most of the 25 move lines I know are the straightforward lines 

alright immma out XD

What r they?