To make a full sew-in, cheap brazilian hair 3 bundles are recommended, since they are cheap bundle deals and 3 bundles are enough for a full sew-in. If you have any questions about the number of hair bundles you need to use, you could watch this video, which may answer your questions.
How many bundles for a sew-in depend on the fullness, the size of the head and the length of bundles. If you want to do full sew-in with closures, then anything over 22 inch requires more bundles. The longer the length, the more bundles you will need. You should never get less than 2 bundles of hair if you plan to do a full head sew-in. Keep in mind the more bundles, the bigger your head will be and the heavier your head will feel. If your bundles are 18 inches and under, 2 bundles of hair with closures are enough for a full-head sew in. 3 hair bundles are recommended if you plan to do a full sew-in. But you can use more hair bundles if you desire big hairstyles.