I am not sure how much NBANDS should I consider while doing the optical calculation in VASP. Cam anyone please explain here how much one should consider.
I would usually use about three times the default value, if I could afford it. But the "real" answer here is that you should test a value of NBANDS, perhaps twice the default. Then increase it by 20% of the default and test it again, and keep doing this until the optical property that you care about converges to within a small error tolerance. That's the way to be sure you are using enough bands.