Yes, you can definitly grow a sunflower from a sunflower seed. The whole reason the sunflower produces those sunflower seeds, is so that when the seeds mature and fall off the flower, they grow new...
If you are wondering how many sunflower seeds are on a sunflower, it depends on how big the sunflower is and its width. You can't just count how many are on there because every sunflower is different.
When the heads are ripe, cut and tie them together and dry in the sun for a week or so. Then you can remove them by either rubbing them out or by laying them on a sheet on the floor and hitting them...