How many sex partners is average?

In general, most people tend to keep a single sex partner, especially as they mature (+27 years old). Younger people (especially true for the western world) seem to have more sex partners. In the end, the answer would be bogus and subjective, since the collection of data is very difficult to obtain both because of the quantity of people involved, the accuracy of answers and the difficulty of having people disclosing that kind of information.