I'm 16 years old, so how many in-app purchases can I make in 1 month (Japan server)?

In the Japan server, the amount of in-app purchases that can be made in 1 month is determined by your age.

Under 16 years old: ¥5020/month
16~17 years old: ¥12,000/month
18 years old and over: Unrestricted

In the game, your age will be increased by 1 year on the day after your birthday.
Your available monthly in-app purchases will be reset on the 1st of each month at 4:00 (UTC+9).