I dunno first hand, I think the most pistols I’ve done is like 15 or 16 in a set. But most people I’ve seen that do 20+ will sacrifice form in some way. Usually the free leg will be bent or something like that. From what I remember of Al doing high rep pistols, he doesn’t pause at the bottom, which allows him to utilize the elastic energy on the rebound and is more efficient. That will let you do more reps. Ultimately though you want to make each rep harder, which leads us to advanced variations such as weighted pistols or pistols with your hands behind your back. So generally I would say rather than trying to do as many reps as possible, strive to tighten up your form. Also remember that shrimp squats are totally worthwhile, and the jumbo shrimp squat is a humbling challenge, to say the least.