I suppose my answer would have to depend on what the goal of the practice is. If you are looking to drill a move, I would say that a one-on-one private technique session would be very valuable. Your instructor (if he's any good) will know not just a multitude of moves but also he will know every knook-and-cranny of the move.
In a private class the instructor will show the move, and you will see the move, and then you will do NOTHING THAT LOOKS LIKE THE MOVE. So the instructor will patiently break down everything "put your head there", "put your shoulder pressure there" etc.
That being said, public or "open" practices with 20-30 students are definitely useful. When you have classes of 25-35 students, the instructor will show the move, and you won't learn quite as much technique, but then during the sparring sessions, you will have tons of people who are in your weight class and skill level. You get to try the new moves you know, and old techniques you know against different opponents who react to your movements differently. It's honestly, best to try moves on people who are worse than you, beneficial to get your butt kicked (because you learn from those who are better), and it's best to go with people your skill level.
All-in-all, I personally prefer larger classes, because I think you learn the most during live grappling/sparring sessions.
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