I'm wondering how many problems are needed in order to learn a subject, let's say Calculus of Several Variables. We know that the professors often assign us a list of problems to solve as homework, but I find that this is often insufficient.

I have found that solving all problems on a book or two is enough to handle (and understanding) most of the problems on a given subject and but I'm wondering if there is some more clever method to learn something because it is not always practical to do all the exercises of all subjects that we learn as math undergraduate students.