On June 18, 2016, Michael Steven Sandford was arrested at a Donald Trump presidential campaign rally in Las Vegas, Nevada in the United States after he attempted to seize the pistol of a Las Vegas Metropolitan Police officer providing security for the event. At the time, Trump was the presumptive nominee of the Republican Party for the 2016 United States presidential election. https://en.wikipedia.org/wiki/Attempted_assassination_of_Donald_Trump
Here the first one is the Sandford one, and the only attempt mentioned on Trump's life.
Which suggests it's been the only one. Ok, one other on Sandford.
Why isn’t the assassination attempt on Donald Trump bigger news? By Callum Borchers June 21, 2016 [...] The most obvious explanation is that Sandford doesn't appear to have come particularly close to completing his alleged mission. He didn't even succeed in arming himself at the Trump rally. Sandford's plot seems to have been feebly unsophisticated; he told authorities the extent of his training was a visit the day before the rally to a gun range, where he fired 20 rounds from a 9mm Glock pistol, a common service weapon, to learn how to use one. https://www.washingtonpost.com/news/the-fix/wp/2016/06/21/why-isnt-the-assassination-attempt-on-donald-trump-bigger-news/?utm_term=.498273f4363b
This one involves a threat, which am guessing there could have been a few of.