I have given a 0.5 cylindrical power in my left eye with 20° axis. My question is should I need to wear a spects.

Depends. If you otherwise see well enough without spectacles in with both eyes, you likely can just monitor your vision. Astigmatism tends to have more symptoms of blur of too high and especially at nighttime. 0.5 cylinder at 020 degrees axis is low order, but oblique in nature, which also tends to be more symptomatic. Have your eyes monitored. You can always wear spectacles if your vision is blurry. .