It ultimately depends on the style that you're going for. Some styles can be achieved with only 2 bundles while others simply look better with 3 or more bundles. The rule of thumb has always been: The longer the hair, the less hair. Which simply means that the longer the length of the bundle, the less hair the bundle will have. So for longer lengths of hair the more hair you will need and for shorter lengths the less hair you will need. This is simply because the longer the length the shorter the weft length. The weft length is how long the bundle unravels.
If you're going for a natural look with a frontal, closure, or a traditional/basic sew-in and you don't wish to go any longer than 22 inches, 2 bundles will give you your desired look.
If you're going for a bob, 2 bundles will give you your desired look. You can use bundles shorter than 18 inches when going for a bob look to prevent your hair stylist from having to cut an excessive amount of hair. Also keep in mind the shorter lengths, the more hair. Make sure to consult with your hair stylist first before deciding which length to purchase for a bob.
If you're going for a natural look with a frontal, closure, or traditional/basic sew-in but you wish to go longer than 22 inches than 2 1/2 bundles would be ideal for you.
For full looks, you should always use 3 to 4 bundles. If you want to add fullness use 2 long lengths that are the same length and layer the shorter lengths. For example instead of using 24",26",28" use 26",28",28". If you're wearing 4 bundles you should wear 24",26",28",28". It will make the overall hair style fuller. If you're wearing a frontal or closure and you're going longer than 22 inches, keep in mind frontals from most hair companies only go up 22 inches in length. Therefore if you're wearing bundles longer than 22 inches with a 22 inch frontal, the frontal will not add fullness to your ends so keep in mind to use your bundles to give you your desired fullness. When purchasing from The Bambi Lifestyle Hair Collections I suggest anything longer than 18 inches, you should wear 3 bundles or more. So just keep in mind when purchasing bundles to consider the style and desired look that you're going for and consult with your hair stylist or hair extensions provider. I offer free online hair consultations, if you have any questions!