Multi-row insert vs multiple single row inserts
I think @a_horse_with_no_name is right about that the index is updated once per statement, because if the statement has not completed its execution the data will not be visible since it is in transaction. And the definition of a statement includes having multiple values

And accoriding to the docs here index creation/update works more efficient with batches than single statements.

There is also a nice article from Peter Manis about the performance of various Insert methods which I can recommend.

Another thing to keep in mind is the FILLFACTOR of the index as it does have an impact on performance as Fabien Coelho describes here.