Expect it
Free events commonly see a large share of registrants not attend. Plan capacity accordingly or use a waitlist.
Send reminders
A reminder a day before and a few hours before, with the venue map, recovers many forgetful attendees.
Charge a token amount
Even ₹49–₹99 changes behaviour. Consider making it redeemable at the venue.
Use the waitlist
Release returned seats to the waitlist automatically instead of running over capacity.