The amount of hair extensions that you would need for a full head would depend on a couple of things. Firstly, the quality of the hair and how mand tip at the end to hold them all together. Many strands are in each pack - for example, some companies would supply extensions where each pack contains 50 hair extension strands, all of which have the small, flat keratin bonds at the tip which hold all of the strands together. Using this basis as an example, a full head could take between 100-180 hair extension strands.
The second factor to consider is the look that you are achieving to desire and how thin or thick the natural hair is. For someone looking for a very full head of hair then more packs of the hair extension strands would be appropriate.