12% of voters encountered at least one problem in 2016.
"If we combine the experiences of all voters, regardless of the modes in which they voted, then we estimate that 12% of voters encountered at least one problem in 2016—compared to 7.7% in 2014, 15% in 2012, and 11% in 2008. In an electorate of 138.9 million voters, that means that approximately 16 million voters encountered a problem voting, compared to 19 million in 2012.”
Source: http://www.legendsvote.org/wp-content/uploads/MIT-Charles-Stewart-Voter-Turnout-Study-2016.pdf