Dave Hopla, famous NBA shot doctor, worked with most NBA stars during the early 2000s (worked with Kobe, was shooting coach for the Bucks in late 90s and coached Ray Allen, still does til this day).
he makes 26 in a row in one minute. notice in the video his form is literally the same every time and you can tell he's a major perfectionist. that's what it takes, be critical of your technique, set goals for yourself and then repeat ad nauseum.
Also a great way to be REALLY critical of your shooting is to only count swishes. this is what a lot of pro players do during their drills, they only count pure jump shots as a way to reinforce what they are practicing for. Dave Hopla talks a lot about this too, he only counts swishes when he does drills. I've done this before and it really helps to picture it that way.
My highest ever is 34. I consider myself a pretty dead-eye FT shooter (something that was drilled into me since I was very young) and tbh most of my players are. I really hate to play lineups with guys that are a liability at the line. In my books, getting foul at the rim and missing the layup and having a poor free throw shooter have to shoot 2 free throws is as good as a stop to me. I never want to be that liability so I always practice my free throw shooting.
What I do in practice is this: I'll do mid range shooting for 10 minutes, then shoot 20 FTs (i usually only count swishes, if I make it but it rattles in it's a redo, some people count it as a miss, whatever the case, depends how meticulous you want to be about it). I try to aim for 15 and above (again only counting swishes, that's pretty hard). If I don't get my goal, by however much I missed it by, I run that many suicides. Then I'll do 3 pt shooting and repeat. It's great for getting in shape and great as well in terms of organizing your workout, really helps a lot for me.