There is no official number NICET publishes, so people tend to either way undershoot it, doing a handful of questions and hoping for the best, or way overshoot it, grinding hundreds of questions with no real plan.
A better way to think about it: you want enough volume to see the same concept tested from a few different angles, and enough repetition that the format itself stops being a distraction on test day. For most people that means at least two to three full length practice exams at your level, taken under timed conditions, not just casual review.
Timed conditions matter more than people expect. Level I gives you about 65 seconds per question, Level II about 66, Level III about 64. That is not a lot of room to second guess yourself, and the only way to get comfortable with that pace is to actually practice at that pace.
After each practice exam, the value is not really in the score. It is in going back through every wrong answer and figuring out whether you missed the concept or missed the wording. Those are two different problems with two different fixes.
Ignite Prep's practice exams are built in full length, timed format for exactly this reason. See practice exams by level.