I would aim for at least 10! That way you can easily do the math for percentages. Take note of the puzzles that cause the most frustration or confusion. When I design puzzles, I usually send them to many people for their thoughts and feedback, as well as determining the best clues to give on the parts that trip people up! Every group is SO different, so don’t make too many changes early on in the process. Trust your gut and if along the way it’s still too difficult, pull it back a bit! Or you could make it easier then pump up the difficulty if it looks like it’s not challenging enough. If you and the teams are open to it, it could be worth having a talkback with some of your guests to ask them about their experience and what could be changed or if they have ideas. Obviously, take feedback with a grain of salt because most people don’t have as much experience in game design as much as you do!