I took 21 AP tests total. I took 3 my sophomore year and 18 my junior year.
I have a ton of info on my experience here: http://emilyxie.com/category/school/ap-tests/
However, I would suggest that students take AP tests when they are comfortable. It can be a very stressful experience, I know it was for me. I would say start small, and if they do well, take more or the same amount.