I prefer a couple extra gores vs too few. I typically sew even my 12" drogues with 12 gores (both for aesthetic and extra strength in case of a very late deployment). However, 6-8 should work just fine. As Handeman said, the more gores, the more round and even your canopy skirt will be, more closely approximating the ideal shape. I've done as many as 24 gores in a 24" (Apollo orange and white), 36 gores in an 8' (my avatar) and as few as 16 in an 18'. The 18' recovered a 130-lb airframe just fine.