The quantity of the henna powder you need depends on hair length & texture.
Below is an approximate idea of how much you will need per application:
Short Hair (above ear) 50-75 grams (4+ tbsp)
Mid Length (shoulder) 75-100 grams (8 tbsp)
Full Length (lower back) 100-150 grams (full jar+)
Hip/Waist Length 150-200+ grams (1-2 full jars)