Need to upgrade to 2018

I need to upgrade one of the office computers to 2018.
The Upgrade Page asks for:
Serial Number
Authorization Code

How do I obtain these?
Do I have to buy a new seat every time we upgrade?

You need to put in the serial number and authorization code from your previous version. It’ll guide you and show you what the charge will be to upgrade.

If you have a multi-seat license for SU2018 already, you’ll need to add the appropriate information.

See this link: License Manager | SketchUp Click on the Frequently Asked Questions link at the bottom of the page.

I have the serial number.
but the Authorization code was lost

Follow the link I provided above. Fill in the serial number and the e-mail address used to purchase the license.

The link explained everything