Go to the the EAMCET official website of which state your preference that is there TS EAMCET for AP EAMCET and enter your details such as your inter hall ticket number your mobile number and your date of birth to re get your registration number.

Actually whenever you register to write the EAMCET exam and attachment with your application form and registration number will be sent to your Gmail which you have provided while applying for the EAMCET exam so you can go to your mail and check it to get your registration number.