When you get bud its made up of tiny little seed pods. If the plant is pollinated all these little seed pods grow a seed. When growing a plant for seed you want to let it mature for alot longer than if you were going to harvest it for smoking. This ensures that your seeds are mature, which inturn insures that they are healthy seeds, ready to germinate into healthy lovely plants.

It all depends on size of the plant, and how much of the plant gets pollinated, but you expect up to a few hundred if the plant is big enough.