Why is the prime of the Forte number 5-20? See Forte number in Wikipedia. First of all, let me say that I know very little about music set theory... I am just curious why a set containing an obvious non-prime (8) is considered prime? Also, is there a formula for calculating those numbers of is it just stochastic brute force? thank you!

The use of the term "prime form" in music theory is not related to the use of "prime" in mathematics, so it doesn't make sense to evaluate statements about prime forms in music theory using the definition of prime from mathematics.