There's no magic number, it's just the higher the viral load is, the more likely it is to infect you. But to give you some context of the possible range, newly diagnosed guys can have a viral load in the millions. Personally, I only had a viral load of ~260,000 before my body reduced it to 2000 before beginning meds, and I somehow didn't infect anyone while I was at the peak of my infection. But of course, there are things besides just a guy's viral load to consider that makes it likelier to get infected. Barebacking (of course), bottoming, having the poz guy cum insided you, and keeping that load inside you for as long as possible all increase the likelihood of getting infected. Being infected by another sti (like gonorrhea, chlamydia, etc.) at the time also increases the likelihood of getting HIV, as well as having an irritated anal lining such as what you might get if you douche with soapy water. Hope this helps.