First off: Best of Luck!!!
For me, my mom taught me grades K-5 (actually 7 but by then....) from texts that they use in China (countless years of pain and torture, blood and tears) and I did get a 5 on the exam (took it this year)
Character Quantity: 10,000 (?)
Now I over-answer ur question
*know the format! I barely read the book since I was cramming for AP Env. self-studying it over 2 weeks but I still remembered the format of the test perfectly
*be culturally-apropriate, I remembered 5 cultural stories the day of the test and used none of them, just incoporate culture into your speaking parts (Ex. be humble in your language use)
*memorize a few idioms and go all out crazy to incorporate them (I incorporated one in the writing)
Text: get a Chinese text, at least 3rd grade (paper back, real cheap, not very thick, and you might be able to find it on amazon, get ones w/out English in them... at all) and Barron's review guide (very thorough but focus on the authentic Chinese text, parts of the Barron's text is hard to fit together since it just gives you lists of characters to memorize)
Practice: DEFINITELY do the practice exams (Easily found via google) and familizarize yourself with the exam (I practiced the day before and the listening, even for me, was *hard* since it was soooo fast and you barely had time to think)
My results: like I mentioned above, I got a 5 (and by the time I took it, my Chinese knowledge probly went down to 3rd grade level) and so did my friends who took it
(5 people all got 5s including me, but the other 4 were "native speakers")
Once again: good luck! the curve isn't horrible so relax, and do well!