I am 21 now. will any height supplement be useful ? i'm 5'6"

No: Once your bones have fused a few years after you go through puberty, no further increase in height is possible. Do not waste your money on supplements that promise this, they will not work. Most males stop growing at between 16 and 17years of age.

Not useful: A 21 yr old is not growing taller if his growth plates have fused. If not fused, he will grow a tiny bit (pediatrician's growth chart for him can tell if his growth has stopped). To look taller, one can choose properly designed clothing and shoes, plus have good posture. Vitamins, herbs, "special" foods, "height" supplements, extra exercise, etc... will not make a normal, healthy person taller.

Maybe: Human Growth Hormone supplementation is ethically appropriate when a deficit of GH is proven by testing. It works best in pre-pubescent children. At age 21, if you tested deficient, some physicians would treat. There are many 'tricks' of diet etc that are intended to increase native GH production. A good diet and aerobic exercise might help somewhat.