Don't much care what your "guru" comes up with, the "number" itself is a modifier of the noun it enumerates, and the noun is what modifies the verb. Your guru can't change that!

If you were bad enough at writing that you said "a number were", you're actually saying "a number (of things) were". The verb is modified by the noun, implied or otherwise. The word "number" has no magic attached to it!

- Pfranz