have a macOS Catalina 10.15.7, How do I get "Numbers" for that version

How it used to work, and maybe still does, is that you can download it from your list of purchased apps in the Mac App store if you had "bought" it sometime in the past. If this is your first time trying to get it for a Mac, though, that method does not apply and I don't know if there is a way to get it. However, even if you are able to download an older version onto your Mac, it is highly likely it will not be compatible with the version on your iPad. If it is an older version, it may not open the files created on the iPad. If it is a newer version it may "upgrade" the files and you will be unable to share them with your iPad again. The common denominator across all platforms is Numbers for iCloud. You may have to use Apple's online version of Numbers if you want to be able to use your files on both devices, and you might need to use it on your Mac in any case.

I am sorry it is this way. The lack of backward compatibility can cause a lot of pain. As a side note, there have been more than a few posts about someone downloading an "update" to Numbers on their newest device and breaking the compatibility of Numbers (and their documents) with all their other (older) devices that cannot get the same update. I have purposely not updated the OS on my newer Mac just so to be sure I don't accidentally install an "update" to Numbers on it and make my files incompatible with my older Mac that cannot take a newer OS or updated version of Numbers.