This is a tough question.
A hollow core, interior door, you can get away with two hinges. It's just a really light pine frame covered in vaneer and if not completely hollow, it's filled with the equivalent of folded cardboard.
A solid core, steel exterior door should have 3 hinges. This is usually galvanized steel over a cedar or fir slab for fire protection and solidity. It's pretty heavy.
An 8′ solid oak door has 4–5. Oak is a very dense wood. This will weigh more than a comparable steel covered door.
A solid steel security door, 2″ thick might have two hinges, but the hinges will have 1″ pins, and are welded to the door. It might have a reinforced piano-type going running up the full height of the door.
Ultimately the number of hinges is dependent on the question of how much weight each hinge will hold, and how many are required to prevent sagging and pulling from the frame material over time.
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