How many U.S. presidents were elected by a minority of votes?

It depends on what you mean.  There were five Presidents who were elected despite receiving less of the vote than their opponent: Trump, George W Bush, Benjamin Harrison, Rutherford B Hayes, and John Quincy Adams.  (Although in Hayes' case, there was massive terrorist violence and voter suppression against African Americans in the South so he almost certainly would have won the vote in a free election). 

There have been more Presidents who have gotten the plurality of the vote, but not an outright majority, including: Bill Clinton (both times), Richard Nixon (his first time), JFK, Harry Truman, Woodrow Wilson (both times),  Grover Cleveland (both times), James Garfield, Abraham Lincoln (his first time), James Buchanan, Zachary Taylor, and James K Polk. 

None, they all won the state vote, which is how our system is set up.

5 (John Quincy Adams, Benjamin Harrison, Rutherford Hayes, George W. Bush and Donald Trump)

we pretend............ and are told everyday that mail in ballots were real...  and we all know it to be false.

I assume you mean that they didn't win the popular vote, just the electoral college votes.

If that is the case, then the answer is 5.

Mail in ballots have been used for decades now.  If somebody can tell me how they think they can be tampered with, then please tell me.

... or, the same vote over and over?