The American Institute of Certified Planners exam has 170 multiple-choice questions total, with only 150 counting toward your score. The other 20 are unscored trial questions the American Planning Association is testing for future versions of the exam. You get 3.5 hours to finish, which sounds like plenty but tends to feel tight once you hit the longer scenario-based questions.
Something worth knowing before test day is that you cannot tell which questions are scored and which are the trial ones. Because of this, treat every single question on the American Institute of Certified Planners exam like it counts, since guessing which ones do not matter is a losing strategy.
My honest tip is to pace yourself in blocks of about 50 questions, so you always know if you are on track time-wise. If you want a breakdown of the exam format alongside a study path, the certboosters page covers that pretty clearly: www.certboosters.com/american-planning-association/path/american-institute-of-certified-planners