9 children, because each one has only one sister...

Eigth sons and one daughter=9 children.

8 son and 1 daughter = 9 children.

NINE. each son has one sister. that means the friend has One Daughter and Eight sons.

He has in all 9 children

9 children--> 8 sons + 1 daughter

Total 9 children

My friend has 9 children including 8 sons and 1 daughter