I've bought extensions there, I got the 18 inch. No you only need one pack and you have to buy the clips and sew the extensions yourself. For the kind I got there anyway. Just ask the lady that works at Sally's and she'll help you and hook you up with everything you need :)
To find out if you can buy more than one pack of extension at Sallys, go to http://www.sallybeauty.com/ and contact them for details. Sally Beauty offers over 6,000 professional quality products for hair, skin and nails.