Some people say to do all the problems, while others say to do a partial number. I would say just do as many as you can. Don't set a limit to the number of exercises you do, just do enough to the point you feel content. If you feel as if the number of exercises you did is good enough, then move onto the next section. If you ever feel like going back and completing some more exercises, then that's also fine as well. However, I do think that mathematics builds on itself. I think you need to do a certain number of exercises that allows you to be "fluent" in the particular mathematical subject. Skipping many exercises is terrible for building your skills in maths.