That's 9% of the entire U.S. population of 303 million people. Of adults eligible to vote, only 14% voted in the primaries for either Donald Trump or Hillary Clinton (about half the voters chose other candidates), as demonstrated in this nifty New York Times infographic. Though that stat seems surprisingly low—and seems consistent with both candidates' historically high disapproval numbers—it's actually similar to the percent of Americans voting for party nominees in previous election cycles.
The most depressing statistic of all: 88 million eligible adults (or fully 44%) do not vote at all, even in general elections.
In the illustration below, each square represents 1 million people: MB