An acceptable number... Only counting the people you sleep with? I personally think if you've had 2 or 3 partners in a year, every year, then that's a huge amount. But if it's 2 partners in 10 years and then 15 partners in 1 year after that, that would seem a lot worse to me even though the amount is lower. It all depends on what a person feels for his partners: love, honesty, respect, etc. It's not bad per se to have had many partners. But I personally would question somebody's capability to bond & truly love if that person has had 15 partners in a year, for instance.