Most juniors shooting for selective schools are incredibly busy with AP classes, ECs and so on. The math on the SAT and ACT doesn't really go beyond Algebra II, which most juniors will have finished a long time ago. Ideally, you test summer before junior year. If the score hits the target, great, done. But otherwise, you identify the weak parts, work on that, and take it one more time for the superscore.
But taking the SAT/ACT 4,5,6 times is usually an inefficient use of time for really busy and stressed out juniors and seniors. Better to study and prep the summer before junior year and get it done and out of the way. And if needed, prep one more time now that they know their weak points.