That's the number picked by the most users of the cheating website SeakingArrangement in a (non-scientific) survey as the ideal for men and women. Nikki Howarth, a party planner, opined on the survey to the Daily Mail: "You want to date a man who is confident and knows what he is doing sexually." But for women, it's different: "My advice to any girl is: don’t tell, particularly if your figure is above ten. Most guys will think worse of you."
When AskMen surveyed male readers on how many sex partners made a woman promiscuous, the most popular answer was 5. But that was the lowest number available — it's possible that given the choice, guys would've gone even lower.