Is Calculus BC an extension of AB to prepare for a single test or will I take both an AP Calc AB and AP Calc BC test? I am trying to get to eight AP’s by the end of the junior year so I can qualify to be a National AP scholar (so I am hoping for the latter). Thanks so much.
You cannot take both exams in the same year. BC has an AB subscore. However, if you take AB your junior year, for example, you can take the BC exam your senior year.
If you take the Calc B/C test, you get two scores (B/C and A/B subscore). I believe you can count each separately, so you get 2 scores for the price of one! This is from recollection; don’t take mine as the last word on this.
I agree with @ItsJustSchool ! Pay to take the Calculus BC exam once you complete BC. You will get an AB subscore that can be counted! 
Two for the price of one! It’s a BOGO sale!
Count for what? The subscore will not count for any AP awards, and colleges won’t double dip to give credit for both AB and BC.
@skieurope, I am not so close to it as you are, so I will defer to you. I believe in getting a distinction such as “AP Scholar with Honors” that the Calc B/C test results will count for both the Calc B/C score as an AP test towards the “Number of AP tests” at or above a score (as well as in the average), and the Calc A/B subscore as a separate AP test score in this calculation.
I am certainly not the final authority on this; but it is how I recall it.
@ItsJustSchool The AB subscore does not count for AP awards. The way that colleges use the AB subscore varies.
OP, it is very possible to self-study for some of these tests and get a 4 or better. You could then vie for the (useless) distinction of National AP Scholar. I may know someone who cuddled up to an AP Stats study guide for something like an hour without ever having taken a formal Statistics class (but having taken some gnarly math- deep and wide) and was able to achieve a 4. If you want to throw down $100 at the chance of a 4 or better, think about what subjects you could possibly self-study rapidly. And the above posters (of course) are correct:
http://professionals.collegeboard.com/k-12/awards/ap-scholar