Hello Yash, hope you are safe and healthy!

I have no idea which entrance exam's registration no. you are talking about. But don't worry.

If your application was successful, then you can find your registration no. mailed in your registered e-mail. Also, your registration no. maybe messaged to your registered mobile number. You can check both and find it. Otherwise, there may be a portal in the website where you applied for the exam, where you can retrieve your application no. using your name and date of birth(this method does not apply to all entrance exams). So, try finding your registration no. in your mail inbox or messages in your mobile.

Hope this helps:)