He has married Marge twice (in fact, three times, but the second didn't count) and has married Amber (the waitress from Las Vegas) once.
Homer married Marge "before" the beginning of the show. They then divorced and "re-married" in the season 8 episode "A Milhouse Divided". Twelve years later, in the season 20 episode "Wedding for Disaster", it was revealed that Reverend Lovejoy didn't have a valid license to perform weddings at that time ("A Milhouse Divided"). They then re-married officially in the same episode ("Wedding for Disaster").
Homer also married Amber in the season 10 episode "Viva Ned Flanders". They "divorced" in the season 13 episode "Brawl in the Family".