i can remember about 30 moves deep into some sicilian lines. it also depends on how crazy the lines are. if they are super complex i tend to remember a lot less.
How much theory can you remember at a maximum

Not much, say 12, I mostly calculate even in the opening or they are pretty simple lines.
WCM Rui Yan Yang won a game of more than 30 moves atleast by playing theory till the very last moves in US Girls Chess Championship against Thalia, I believe.
Once I prepared a theory just before the not so important game, chess.com rated with random person, and forgot the line after move 4 or 5.
I am still very new to this theory thing as I started it around March this year and not doing it at a big scale at the moment.

Depends on how long I try to memorise.
+1
We can recall entire famous game(s) if given the time to memorize them, but with opening theory it is tougher because of all of the sidelines and possible mainline branching variations. When memorizing a game, you really just have to memorize the game moves and maybe a few obvious alternatives. Learning opening theory is much tougher.
Addressing the original question: I don't know too many openings any deeper than 10 moves or so - maybe 15 on the most theoretical ones.

Just a test
Around 20 moves Caro-Kann (B-19) as I played this lines thousand times in bullet.
My theory means, "copying/memorizing what Stockfish play".


Why would I bother counting that’s a waste of time
Exactly! I'm very lazy to count em

It doesn’t matter if you know theory unless you know why you play the move
if you play 20 moves of theory and then you have no idea what to do that theory didn’t help you much
Just a test