The first number is the Number of Sexual Partners.
If each of your partners has had only the same number of partners as you had at the time you had sex. The second number is the Number of People You Are Exposed to:
Explanation of Chart
For this calculation, we assume that each time you had a new partner, they had only had as many partners as you had. So when you had your first, we assume you were his first. You now have one person who has only had you as a partner. Add to that a second partner who is in the same situation as you--only having had one prior partner. Now you are exposed to your first, your second, plus your second's first. Then bring in a third who is in the same situation as you. You are now exposed to your prior partners plus your third's first partner, your third's second partner, and now your third's second partner's first partner. See how it works? A little sex goes a long way!