You have not given us a lot of information to work on but, assuming you have previously installed and registered the software you will find the number in your Magix account, once logged in, under My products.
OTOH, if you have not installed the software (is this Sound Forge Pro or Sound Forge Audio Studio, btw?), where did you buy the software? If direct from Magix, you would have received an email confirming your purchase, which would have included your serial number. If you bought from a third-party seller you need to ask the seller about your serial key; Magix will have no record of your purchase and so cannot help you at all.
BTW, please don't post in bold! It is considered the forum equivalent of shouting!