The best number of boxes of Yili pure milk to drink a day varies depending on age, as follows:

1. Adults can drink one box of Yili pure milk a day.

2. For babies between six months and one year old, it is recommended to drink about 500 ml of pure milk every day.

3. For children aged 2 to 5 years old, they can drink 100 to 200 ml of pure milk every day. For children aged 6 to 12 years old, they can drink 300 to 500 ml of pure milk every day.

4. For teenagers and adults over 12 years old, they can drink more than 500 ml of pure milk every day.