If you have purchased the download version of the software then you will have received the serial number in an email.
If you have lost your serial number and are reinstalling previously registered Music Studio, then go to the magix.com website, click top right blue icon of the home page, login using the email address and password you used to register the software. Click My Product on the next page. This will take you to a list of your registered products and serial numbers.
You have posted to a thread which is nearly four years old, with a question that is really nothing to do with the subject raised by the OP. Please do not do this, but if you have a question to raise which we, fellow-users, may be able to help with, please start a new thread.
However, in your case, no-one here has any means of assisting you since Magix staff very rarely read or post. You will need to access your Magix account where you should be able to redownload your software if you need to do so and find your serial numbers (assuming you registered the software with Magix when you first installed it).