While an audience of 2.5 million for a TV show would be regarded as disastrous in the broadcasting world, in the cable world it can represent a huge success. That's the number of people who tuned in for the season finale of Showtime's Dexter at 9 00 p.m. on Sunday, but Showtime, the No., 2 pay-TV cable network behind HBO, pointed out that the initial airing does not tell the whole story, since audiences have multiple opportunities to see its programs. At 11 00 p.m. that same night, Dexter drew another 401,000 viewers, and by the time the accumulated audience figures are tallied for all of the repeats plus DVR playback, Showtime figures that a record 6 million viewers will have watched the show. It said that Dexter averaged 5 million viewers per week this season, a record for the show, which just concluded its fifth season.