This absolutely depends on the student, but you should also consider the specific balance of AP courses and which AP courses instead of solely the number of APs. I'll offer a few questions you and your student could consider for this issue:
1) How well is your student doing in school already? If your son or daughter is doing well in school, but has a mix of As and Bs, one AP course in sophomore or junior year may be the best choice to start with. If your student is doing extremely well, more APs may be good to challenge her.
2) Does your student have a subject area in which he or she does better than others? If your high schooler is great at math, taking more math and science AP courses, and regular English and history classes might be the right path. She might be able to manage 3 AP science and math classes better than 1 history AP, for example. If she is doing well overall, consider the first question and think about how many she feels she can handle and pick her favorite subjects. I'd advise against taking APs in subjects that students really dislike because it could easily become a burden.
3) How many AP tests will your student take? If your student takes five AP classes, that's a lot of tests to be studying for at the end of the year. They also cost about $80 a piece. The tests aren't required, but are often a good idea. Not every college accepts AP credit and many accept them as general elective credits, but with the AP credits I took in high school, I was able to graduate a year early with only a few extra classes in my schedule. This saved a year's worth of tuition, which was well-worth the investment of the $80 per test. Although, I never took more than two tests in a year. Think about how many tests your student will be able to handle in May -- and what other events they'll have going on then -- when contemplating how many APs to take each year.
Overall, I'd say taking one AP class as early as your student is able, is a good starting point. He or she can get an idea of what's involved in an AP class, the difficulty level, and increase his or her course load the next year.