How many fascist Presidents have we had?

[Scam of God]

There's a difference between fascism and authoritarianism. Read Bill Paxton's The Anatomy of Fascism: fascism can be pretty accurately described as a revolutionary form of conservatism that relies on a mass-movement with broad appeal in both the lower and upper social strata to form a vanguardist coalition capable of seizing state power - and promptly bending it towards corporatist, exclusivist economic ends. Its rhetoric appeals to glamorized, idealized symbols of unity and purity. It tends to radicalize before total collapse (Werwulf being the best example, wherein Hitler wanted to employ an extreme form of scorched-earth tactic combined with total-sum mobilization of the German populace to effectively end all human life in the nation before it could be captured).

Not all fascisms are alike, either, and this can be most easily gleaned in their aesthetics (the aesthetic plays a central role in the ideology). Mussolini had envisioned a distinctly modernist ideal for Italy in the 1920's, a sort of art deco scientopia with a high degree of stratification and very little in the way of public support of outdated notions like Catholicism. He was not personally anti-Semitic, and only half-heartedly supported Hitler's demand that he expel Jews from the nation until pressed. Hitler, to the contrary, had a decidedly baroque view of Germany, and he drew on both severe Lutheranism and neo-pagan imagery to create what might be viewed as an organic merger between high German gothicisms and modern technology (an attempt at volkgemeinschaft). Moreover, Hitler hated modernist ideology; Mussolini basked in it.

Don't throw the word around lightly, but don't use it too specifically to refer to Nazi Germany alone. Mussolini and Hitler were fully-realized fascists; Perón's movement included elements of both fascism and socialism; Franco came to power on the back of the falangista movement and crushed it on assuming power; Salazar adopted a few fascist trappings, but generally disliked it, inclining as he did more towards paternalistic theocracy; Mugabe was a penny-ante authoritarian.

As a rule, authoritarian states are usually juntas or otherwise governed non-democratically, with a ruling class interested only in stuffing its own pockets. We have had a few Presidents of this persuasion. Fascism is much more; it is a mass ideological movement. I do fear, however, that today's G.O.P. are inadvertently releasing a highly-theologized form of populism on an unprepared citizenry that could very easily turn into real, authentic, fascism with a religious tinge should the proper conditions arise.   

[Scam of God]

I say four:

John Adams
William McKinley
Woodrow Wilson
George W. Bush

Reagan is borderline but I'll err on the conservative side.

You'd have to include FDR and Truman on that list.  McKinley wasn't a fascist.

McKinley was probably the closest to an authentic fascist that we've ever had, and by this I do not mean a free-marketeer - he actively supported business while President. He was a corporatist through and through. He simply lacked the social authoritarianism that most - though not all - fascists consider a part of their ideology.