Your first question is the hardest. How many dives will it take before you're ready?
My answer is depends. Sorry for the seeming copout, but there are lots of variables so I don't think it possible to give you a number.
It depends on you and how well you take to diving, and get comfortable with the core skills. Usually thats 10-25 dives or so. It depends on how much of that experience is diving from boats. It depends on whether we're talking diving in a lake, sheltered cove, or in Nantuckett Sound.
My suggestion is that after your course, try to connect with a local club, or do some dives with some of the local charters. Gain some experience, especially in diving similar to what you plan on doing, ask lots of questions, observe what's going on, what works, and where the pitflls seem to be.
If you're patient and fairly honest with yourself, you'll know when you're ready.
In the sailing world there used to be (and maybe still is) something called a confidence course. When getting ready to skipper a boat much larger than what you're used to, or skipper for the first time, you'll hire an experienced skipper to join you as an observer, and to be there in case you get in over your head. Afterwards there's a debriefing where you review how it went. When you feel ready, you might try something similar as you plan and execute your first independant boat dive.
BTW- let me know when you're ready, I'm always interested in who has boats to dive from locally.
dF