The answer to this question is "How much do you want"?
There are many different weights of hair extensions available on the market. You can buy lighter volume (65g-85g sets), medium volume (100g-150g) or heavy volume (220g) hair extension sets. Generally the heavier weight sets either have more pieces per set (which means you need to find room on your head for them all! 7 or 8 will fit comfortably) or sometimes more hair per piece (which means heavier wefts which can result in an overall reduction of comfort and may cause extra shedding of your own hair).
Many companies will tell you to 'beware' of lower volume hair extension sets and encourage you to purchase the heavier sets (which of course are more expensive). So ask yourself how much you want rather than just buying what you are being told that you want!
Many women are just looking to double their current volume for more natural looking augmentation. This is absolutely attainable with the lighter volume sets given that the average ponytail circumference is approx 5cm-6.5cm which is the same circumference as a 70g-85g set.
So if you're looking for more natural looking volume, go for the lighter volume hair extension sets, and if you're looking for a far more dramatic Barbie-esque look (or if you have short hair and are looking to extend your length rather than just augment the thickness), then go for the higher volume sets!
Happy Hair Shopping!