You need to be registered before you are sent your application number; this is known as your Personal ID. Go to ucas.ac.uk, click "Apply" and then select "Register/Log In." If you are applying through your school or college, you will need its "buzzword" to register. The process takes around 15 minutes.
All correspondence from UCAS will have your Personal ID number on it. It will be in your confirmation email, sent when you have successfully registered. It is a 10-digit number and appears in the format 123-456-7890.
You can have your Personal ID sent to the email address you used to register with Apply. Go to ucas.ac.uk, click "Apply," select "Register/Log In" and then click "Forgotten Login?" Choose "Username Reminder," and then enter your email address. Your Personal ID number will be sent you.