What do girls think if a boy not talked to any girl during his college or school time?

They would think he is shy maybe. Some girls will speculate that he does not know how to talk to them. But there are others that will ask you to find out.

How are they able to know that person hasn't talked to any girls? They would have to stalk that boy 24/7 to be able to know such things. Which is highly unrealistic. People don't need to have any assumptions. Assumptions in general are pretty bad as it's just a random guess about that person which is most often incorrect. If you specifically tell them, they may not even believe you, so what's the point? It doesn't matter to how many girls you talk. It doesn't give validation. It's a really bad source of happiness to try and have as high number as possible.