I’m currently using PrepScholar to prepare for the ACT. I really like it! It’s an online prepping website. It costs $400 for a subscription,but that’s inexpensive compared to the cost of prep centers like Princeton Review or SAT Professionals. The practice questions are really helpful, and PrepScholar tailors your lessons so that your score improves as fast as possible.
You can’t take the regular SAT and the SAT subject tests on the same day (although they are offered on the same days, if that makes any sense). You can take up to three subject tests on the same day. However… not all subject tests are offered on every SAT date! For example, World History has historically not been offered on every date. You need to go to the college board website and look at the testing schedule. But this is just if the colleges you plan to apply to require the subject tests; probably only the top 50 or so colleges in the country (counting all types of colleges) require them. So figure out if you need them.
If you feel the need to take a class, then I would think an 8 hour class wouldn’t be enough. Certainly my kids self studied for a lot more hours than that! But, of course, longer classes are more expensive. My kids did weekly study sessions on Sunday afternoons of about 2 hours per session for about 6 months.
I agree with @billcsho on treating each attempt as your last, BUT leave enough room on the calendar just in case. We see a lot of students out here fall of their senior year really counting on doing well on that last test date, and stressing out over it.