Researchers at Oxford University conducted a survey asking people to describe their close circle of friends, and compared the difference between people who were in romantic relationships.
On average, when asked to list the members of their core group of friends, people name 5 friends. People in romantic relationships could only name an average of 4 close relationships. That's an average net loss of 1 person in your friend circle as a result of adding a new person into your life. Typically this means that you lose 2 friends from your group of 5 and then add your new lover to get to a total of 4.
This core group consists of people that you see at least once a week. You may of course have many more peripheral friends. Adding a new lover into the mix will force you to budget your time differently, forcing you to lose touch with an average of 2 people in your core group.
Read more about this study here.