Number 100?

Sana thinks she completes her goal with sleeping with 100 girls. Until she re-counts everyone she has slept with and now believes she only slept with 99. Sana fixes that by trying to sleep with another girl right away. Tzuyu comes into the picture frame as (secretly) the 101th girl. But there's a catch, Tzuyu already knows Sana's plan and what she is getting at.