In the Massachusetts primary election on Sept. 1, 6,637 Belmont residents cast to determine the Democratic or Republican nominee for races in the November general election. Of those votes, about 75% were cast through a mail-in ballot, according to Town Clerk Ellen O’Brien Cushman, with many being turned in on Election Day, Sept. 1. Exact numbers for mail-in ballots were not available at the time of publication due to the recount of the Suffolk and Middlesex County State Senate election.…