I think the number doesn't matter as much as his approach to his previous partners. If he knows their names (first name counts), what they want for breakfast and cares at least a little about mental compatibility, he's not a slut.
My partner is a few years younger than I am, and has had about 18 partners (about 4:1 female:male), mostly short term flings. That's slightly more than me, but not a particularly high number. A previous partner had been with 80 girls, and I didn't see him as slutty either. But I've had male friends with lesser numbers who were less particular about their partners. That makes the difference, I guess.
It doesn't really matter to me anyway, though.