How many times can a person have sex without contracting AIDS?
An infinite number of times. AIDS does not arise spontaneously once you exceed some magical sex threshold. AIDS is caused by the human immunodeficiency virus, better known as HIV. It's a germ, like the flu or polio or smallpox. If you never encounter someone with smallpox, you're not going to get smallpox. If you never receive bodily fluids from someone with HIV, you're not going to get AIDS. Now, if you don't know the HIV status of your sexual partner, you are rolling the dice every time you have sex with them. Maybe they have it, maybe they don't. If they have it, maybe you catch it, maybe you don't. The more you roll the dice, the greater the odds they'll eventually come up snake eyes. That's why a large number of sexual partners is considered a risk factor for HIV: It's a large number of die-rolls. But if you fuck a thousand people, and none of them have HIV, you won't get HIV. (Assuming you don't share needles or get an unlucky blood transfusion.)