When you people say “learn an opening”, how many moves are we talking?

we memorize the ideas in the opening. Lol

also you can memorize the ideas of an opening but fail to memorize the correct move order and end up with regrets

Best is to learn how to be a decent calculator. For example, take any game, and go through the moves. In positions that are interesting propose an alternative and calculate (no cheating ) the variations. Opening theory is one thing but knowing which moves constitute the Catalan, doesn't make you a decent Catalan player.

My opinion is that on average an opening is 15moves-ish.  Of course the normal opinion is when the rooks are connected.  It'll vary greatly from different openings. I just consider 15 an average.

memories reside in the hippocampus. calculation happens in the parietal lobe. they are completely different functions of the brain and using one does not preclude the use of the other. we memorize formulas in mathematics and this makes calculation less difficult. positions and patterns are like formulas.

In Kasparov's immortal game against Topalov, Kasparov already saw 37.Rd7 when he played 24.Rxd4!! That's an almost 15 move deep calculation.

I'm guessing everyone already said this, but you want to learn the "why" behind moves. Definitely don't just try to memorize 6 different lines with 17 variations in those lines.

Well, I wouldn't really say that learning an opening is about how many moves you need to know in that opening; it's more about knowing the plan of the opening, knowing how to enact that plan, knowing why the particular moves of the opening are best for that plan, knowing the common plans to counter that opening, and knowing how to counter those plans. You learn most of those things through study, practice, analysis, and review. So, I am saying that what you need to do, is to get a basic understanding of the opening(as in the structure, the move order, and the basic plan) and write it down(I suggest using something like the "library"/"collections" feature on this site). You then should play one or a few practice games to get the feel of the opening. Afterwards, you should look into what common plans to counter the opening and what to do when an opponent tries one of those plans(obviously write it down)--when doing this, a games database(set to "masters"), an analysis board(especially so you can mess around with the position and see what works and what doesn't), and some games played by grandmasters in that opening(especially to see what plans they go with in the opening), are your best friends. You next want to play a bunch of games, and until you feel comfortable with the opening, you want to analyze those games and use the tools I mentioned to see what you did wrong and what you did right. And then every so often, when you don't feel confident in the opening, or when you don't preform well in the opening, you'd want to use the tools I mentioned to re-study/review the opening--it is a good idea to check with an opening explorer to see what mistakes you keep making and what lines you are struggling with at this point. Of course, obviously, you don't need to do all of this, but doing those things can help a lot.