A is the son of B, C. B's sister has a son D and a daughter E. F is the maternal uncle of D. How many nephews does F have ?

Clearly, F is the maternal uncle of D means F is the brother of D's mother i.e., F is the brother of C. C is the sister of B. 

So, F is the brother of B who is A's mother. Thus. F is the maternal uncle of A. So, A and D are the nephews of F i.e., F has two nephews