Laser hair removal acts preferentially on the growing hairs which contain more melanin and absorb more heat. Much of the hair on our skin at a point is in resting or falling phase where there is no or little melanin and thus do not respond to laser. These hairs enter growing phase after every few weeks - hence sessions of laser hair removal are done every 4 weeks or more to target the next batch of growing hair. In areas where hair is thick and quick growing, sessions may be more frequent and higher in number (like lower face) while thin and slow growing hair needs sessions at longer gaps (like chest, abdomen, back).
The number of sessions depend on your skin type, hair type and density and your hormonal status. A minimum average of 6 sessions is recommended to almost everyone for significant results. However, women with hormonal issues like polycystic ovary need more sessions along with oral medicines to reduce hair growth as hormonal imbalances makes the hair resistant to laser. Males and young people need more sessions than older females due to similar hormonal differences.
Visit a dermatologist to get your hair and skin type checked and he/she can guide you regarding the number of sessions you might need for expected results and whether any additional skin care or medicines would be required for the same.