How many different people have served as President of the United States?

How to get close to this number

Divide the span by the average tenure. The office has existed since 1789, and presidents serve an average of somewhere around five years once you account for two-term, one-term and partial-term holders. Two hundred and thirty-odd years divided by five gives a count in the mid-forties.

The trap is the numbering, and the explanation spells it out. The numbered presidencies run higher than the number of people, because a president who served two non-consecutive terms is counted twice in the ordinal sequence. So "the forty-seventh president" and "the forty-fifth person to hold the office" are both correct and describe different counts.

That quirk is a good general lesson about ordinal numbering. Ordinals count occupancies rather than individuals, and any institution with returning office-holders will have an ordinal count higher than its headcount. The same applies to popes, monarchs and prime ministers, and reading which one a question asks for is half the work.