You must remember that there is a mitzvah to have children and many people probably had many wives to have many children.
Firstly, the average number of wives still had to be pretty close to one. After all, there are only a finite number of women to go around. Every additional wife a person took probably meant that someone else wasn’t going to get married. (Unless, of course, you’re going to posit that there were many times more women than men.)
In addition, you must remember that if a man had more than one wife, he had to support them and their children. I would be willing to bet that only the wealthy could have more than one wife at a time.
Here’s a hint: how many tana’im or amoraim can you think of in the Mishna/Gemara that are mentioned as having more than one wife at a time? Very few, if any.
The Wolf