how do I download older versions of numbers for 10.13.6

Apple does not provide full installers for past versions of its Pages, Numbers, or Keynote applications on older operating systems where none of these applications are installed. The current versions in the Mac App Store require macOS 10.15.7 (Catalina) or later.

Follow the alternative advice provided.

Visit http://www.icloud.com/ and open the spreadsheet directly from Numbers website in your iCloud account.

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/