How do you write a good "Why us?" college essay if you apply to more than 10 different universities?

You could start with the actual reasons why you chose to apply to each of the >10 universities.

Of course, this assumes that prestige, ranking, ego, or “my parents made me” are not the reasons.

This topic is meant to elicit your perception of the fit between you and the school in question. How much thought have you really given it? Do you know why you are applying?