28 days if you want an easy answer.
It is generally understood that it takes between 27.3 days and 29.5 days for the moon to complete one rotation around the earth. This is usually rounded up to 28 days. This adds up to roughly 13 times per year.
The confusion arises from the fact that there are two very different ways to calculate the moons orbit, and thus the result is varied depending on which method is used.
The sidereal month is based on it's rotation in relation to the stars, and this estimates 27.3 days
The synodic month is focused on the duration of reaching the same phase, which amasses 29.5 days.
Either way you consider it, both answers are correct, but just to be safe, it is best to go with 28.
I hope this helps.