Dial 121 from your Airtel SIM and follow the IVR options
Dial *121# and check the menu for your number
Dial *282# to display your Airtel mobile number
Call or give a missed call to another phone and check the incoming number
Check the SIM card pack or welcome message if you still have it
Open the Airtel Thanks app and view your registered mobile number
Check your phone settings under SIM or About Phone
Contact Airtel customer care from your Airtel number for assistance