Tom Selleck's reign as CBS's prime prime-time hunk may be more short-lived than expected. "Magnum, P.I.," which returned to television Wednesday with a new episode after eight weeks of reruns and preemptions, has been pared from a full season order of 22 shows to only 13. The decision to shorten the show's life span was Selleck's. "Thirteen is enough," said a Selleck spokeswoman. "He's not tired of 'Magnum,' but of working six days a week for eight seasons." That means only five more new shows before "Magnum" is relegated to syndication heaven. The swan-song episode is scheduled for May, but producers will leave the door open for future "Magnum" specials, meaning he won't be killed off again as he was at one point last season.