Higher quality mountain bike tires are designed to allow riders to select from a wide range of PSI levels. Some models will allow riders to use any any PSI from around 30 PSI up to 80 PSI. The lowest PSI level works well in sand, muck, mud, or loose dirt. The highest PSI is for riding on smooth, paved roads.
Rider weight is an important factor. On the same bike, with the same set of tires, a 120 pound rider might get the best performance at 40 PSI and a 220 pound rider might get the best performance at 80 PSI. A rider's skill level is also relevant. A skilled rider can ride smoothly at lower PSI levels and never get a pinch flat. Gonzo riders who plow full speed into every rock and every pothole can get pinch flats at maximum PSI levels.
So, to find out what PSI level works best for your riding style, weight, and preferred riding surfaces, experiment. Look for a PSI level that allows the sidewalls to maintain a normal profile, yet flex enough to absorb shock while riding over rough surfaces.