At least a dozen. More experience in different genres would be a definite plus.
That's nonsense.
Sometimes, a filmmaker can make a masterpiece with one movie, a musician can create a single perfect song, an author can write one life-altering and timeless book...
They don't have to "prove it" a dozen times over to be any less amazing or influential.
In fact, considering many games can take up to 3-4 years to make, if they wanted to make a "dozen" games with that development schedule, it might be nearly 50 years before they've "earned it" by that "make a dozen" criteria.