A common question I get from clients, "how many bundles do you think I'd need?".
Your head size, the look you're going for and the length of hair you are getting.
In most cases, 3 bundles of hair extensions are enough for a unit. In some cases, you may be able to get away with just 2 bundles along, it simply depends on the length of hair being used along with the look you are going for. For the fullness effect, 4 bundles of hair is recommended.
The longer the hair length, the more bundles you'll need.
Some more key factors would be the look you are going for and how full you'd want your unit to be.
Do not make the mistake of having a 20" - 21" head size, ordering 4 bundles of 12" straight hair for a blunt cut bob, but wanting a "natural & sleek look".
If you have any further questions in regards to figuring out how many bundles to use, contact me, I'll be happy to assist.