I'd store the different email bodies in the config file (probably web.config) and add a parameter to the GenerateEmailOTP method that lets the calling code specify which email body they want, and the method will retrieve the appropriate mail body based on what that parameter is.
private bool GenerateEmailOTP(int randomNumber, string emailId, string emailType)
The simplest way of doing this would be to store the different configs using a settings name like "newPinBody" and "someOtherBody" and the calling code will pass "newPin" or "someOther" as a parameter to the function, and the code will then use ConfigurationManager to read the appropriate body
string body = ConfigurationManager.AppSettings[emailType + "Body"]