Answer: On Tuesday, Barack Obama will become the 43rd person to hold the position of President of the United States.
Grover Cleveland served two non-consecutive terms as President of the United States. He was both the 22nd and 24th presidents, but of course he was only one person. Therefore, the actual number of persons to hold the office is one less than the number of distinct administrations.
Sorry I had to clarify what I was referring to when I meant Presidents (and thanks to Thomas for bringing it up). Most people date the first year of the country as 1776 and the signing of the Declaration of Independence. Still others place it in 1783 with the Treaty of Paris, in which Great Britain relinquished control of the colonies. I, however, see the beginning of the country as 1789, with the ratification of the Constitution and the election of George Washington to the presidency. So I do not consider any leader before Washington to have truly been president.