Portfolio - your selling tool
Your portfolio is your selling tool! The presentation, quality and variety of shots are all important factors and you should choose the 'pick of the bunch'.
Modelling agencies are very busy so fifteen photos is just about right: a few head shots (hair up and down), full-length body shots, lingerie / swimwear, outdoor, sporty and natural.
A portfolio isn't a photo-album, it's a representation of your past work and your professionalism. A good modelling portfolio will be shot over time by numerous photographers reflecting different looks and styles.