Yeah, I think after completing a lesson it's best to do as many as you need to truly "get it." That's always going to vary. But definitely do enough that you understand the ins and outs of each question and how to properly execute a strategy to solve them.
Very hard for me, but I think the key was to go back into the passages and re-read content to verify answers. Unfortunately, this cost me a lot of time and didn't properly answer 7 questions between two passages.