https://en.wikipedia.org/wiki/Faithless_electorTrump could become the first winning candidate to have a faithless elector since Nixon in 1972. Faithless electors since then have all defected from losing candidates (Ford in 1976, Dukakis in 1988, Gore in 2000, and Kerry in 2004).
The last time that there was more than one faithless elector in an election was technically 1912, but that was a unique circumstance since the Republican Vice Presidential candidate died before the election and after it was too late to remove him from the ballot....so the fact that Taft's electors didn't vote for him shouldn't really count IMO.
So you'd have to go back to 1896 to find the last instance of more than one faithless elector in an election in which the candidate to whom they were pledged was still alive....there were four that year, but all for Vice President only, none for President.
The last time that there were multiple faithless electors for President in one election was in 1872, though again that was a unique situation since Greeley died after the election but before the EC could convene.
You have to go back to 1832 to find the last time there were multiple faithless electors for a Presidential candidate who was alive (there were two that year).
The last time there were more than two was in 1808, when there were six.
And so if Clinton ends up having seven (or more), it would be the most since 1796 when the electors cast two votes for President instead of separate votes for President and Vice President.