How many teachers should you typically ask rec letters from during junior year?

Usually, you want 2 teachers (one from humanities, one from STEM) and 1 or 2 other recommenders, like a chaplain, community service leader, etc. Basically, adults that can testify to your character and not just academics. If you have been involved in any important extra curriculars, I would suggest asking the adults in charge of those. A small number of colleges will require 3 teachers, so look at the colleges you want to apply to. Most will allow you to submit as many recommendations as you have (but you don't necessarily want to overburden to many people).

If you already have your recommendations, then there is no reason to ask for more senior year. Unless, maybe, you became like super close to a different teacher and accomplished something spectacular. One other thing you can do, that I wish I had done, is to ask the teachers to save their recommendation as a pdf even if they don't actually send it to you. The Common App will send them a portal where they will write the recommendation in directly. That means if you ever need a recommendation in the future (and you will, either for scholarships or jobs), if they don't have it saved, they would have to rewrite it.