Another question I get from clients, "how many bundles do you think I'd need?". 

The answer is based off two factors: 

Your head size & the length of hair you are interested in. 

Here is a chart on hair lengths in different patterns:

If you are not already familiar with hair extensions, I will simply tell you: 

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 have a 20" - 21" head size, ordering 4 bundles of 12" straight hair for a blunt cut bob, but wanting a "natural & sleek look".