The predicate is also built in Prolog: 

		length(List,Nr).
     
		?- my_length([a,b,[c,d],e],R).
        	R = 4 
		?- my_length([[],[]],R).
		R = 2
		?- my_length([[[]]],R).
		R = 1
 	
		my_length([],0).
		my_length([_|T],R):- my_length(T,R1),R is R1 + 1.