I use 150 as a guide for a standard fitting (0.6-0.8g strands) more is needed if theirs hair is especially thick or short and less if finer. I myself cant handle more then about 120 in my hair, preferably 100, as it feels too thick and unnatural. You also need to take into account the length ratio of the hair your fitting, some cheaper single drawn hair can taper to almost nothing on the ends so doesn't give you a nice full finish without added extra hair in (I've learnt this from experience!) I opt for good quality single drawn hair to get a good length ratio plus I offer double drawn as an upgrade option.