How many presidents have been impeached?

Two.
Nixon probably would have been but resigned the presidency before probably being impeached.
The other two being Bill Clinton and Andrew Johnson.
The Johnson one is interesting because he was impeached for "attempting to remove certain office holders without approval of the Senate" in 1868.
I'm guessing either somebody changed that law (The Tenure of office act)
My research says "The act was found to be unconstitutional" by the Supreme court.

Two, both were acquitted later by the Senate. Bill Clinton was one of them.

Not enough!

Bill Clinton and Andrew Johnston I think..

2, soon to be 3

Not enough

2.5

Personally? 7.

7.0968356e+85

Two

2