where can i Find product key for Office on mac

Microsoft mails it to you if you purchase a digital copy or boxes it if you get a boxed copy with digital download.. Check your email if were the one who purchased it, otherwise you have a product ID which is not your registration number but it may assist you if you have to call Microsoft. If it's on the old drive it's encrypted. MS allows 5 transfers if you own their software, after that you cant do any more.

For future reference, Microsoft has an automated activation call number from which you can easily get a lost key replacement number.