Somebody came in my Apple ID and change mobile number on its own

See this https://support.apple.com/en-in/HT201303

If you had revealed the Apple ID and password and other credentials like questions and answers for security questions , admin password to other person , then only he can change the trusted number , otherwise it's impossible to enter the account .

They could take you personal data if they can log into your Apple ID.

You might want to consider changing your password and making sure it is a strong one. You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Apple ID has been compromised

 Apple ID Password Change

Note the restrictions in the article.

Apple ID Name Change

 What to do after you change your Apple ID or password - Apple Support     I suggest doing this before you change your Apple ID or password.

 Apple ID -Two-factor authentication

 Two Factor Authentification availability

 Apple ID - Using app-specific passwords

Apple ID - Two Step Verification App Specific Passwords

Apple ID - Two Step Verifications FAQ

Log in below and remove their number.

Apple ID - Manage