Formally, is it correct to write:
A number of questions has been asked here.
A number of questions have been asked here.
As a non-native speaker of English, I would prefer the former: the subject seems to be "number", therefore the verb ought to be singular, I'd say. However, the latter seems more common, and therefore I believe that my gut feeling is just plain wrong — but I would really like to have a definite answer.
Moreover, is it the same for "a myriad of", "a plethora of", and so on?