I just got my December SAT Math II Subject Test back and I got an 800. However, it says that I am in the 80th percentile. Did anyone else get this too? Does this mean that 20% of the test takers also received an 800?
Yup. That is what it means. It is still the score you want. 
800 being about the 80th percentile is very common for the math 2 test. The scores are skewed because, umlike the SAT where most everyone in high school may take the test, the math 2 is typically taken by those very good at math, with high grades and SAT scores, who need a subject test because they are applying to high ranked colleges that require or recommend them. Percentiles are not relevant to subject tests, showing proficiciency in the subject is, and an 800 obivously shows that, as does anything above a 700.