I counted more than 30 characters.
Carmen (who first appeared in chapter 7 along with Momoko, Camila and Monica) and her son in Ann's gangbang pages ahead. In chapter 16, Carmen and her son are having sex together. The attendant in the current chapter is with a blonde girl named Dani. Both appear in chapters 13 and 14 where they both talk about Lukas and Carl. There is a brunette girl with wavy hair with Pete in chapter 6 at the beginning, I have doubts if it is Kimberly next to Rory or sitting down. Besides Shona's boyfriend, there are two girls in chapter 3 when Dylan and Rory are discussing incest on the island, two women (blonde and brunette with straight hair) are kissing. They appear twice and did not reappear during the plot. If there are more characters that I have forgotten so far, I have doubts.