The industry standard was always 200 strands for a full head but unfortunately not everyone sticks to this, making it impossible for people to compare prices. I've had clients who have had Easilocks fitted elsewhere come to me with as little as 110 strands thinking they had a full head. I've seen full head, extra thick full head and super thick full head advertised when what they are actually offering is a half head, 3/4 head and a full head.
Your clients own hair should determine how many extensions you apply. You need to look at their density, length, layers, etc. and what they want to achieve before deciding how many strands you need. I would also look at ordering different weight strands depending on how fine/thick and dense your clients hair is. 1g strands are far too heavy for someone with very thin/fine hair.