Eight:

16. Natural language

17. Wonderful cloning

19. Biological invaders

20. The illusion of sunset

Eight times:

1 1. Fear of nature

12. Lop Nur, the disappearing Fairy Lake

13. The mystery of lemmings

14. The return of geese

Nine:

5. Dedication and happiness

13. There is more than one correct answer.

14. There should be a spirit of understanding things.

16. Have China people lost confidence?

Nine times:

9. Talking about life

12. Life