Please check this list and search for your iMac.
https://www.niwtech.com/guides/apple/imac-imac-pro/mac-os-compatibility/
For each hardware level there is a max revision level / version of macOS that can be used on this computer.
The Numbers version is tied directly to the macOS version.
If your iMac is too old you can not update to a newer macOS version and therefore you can not get the newest Numbers version. I would think that Catalina10.15.7 is the latest version that your iMac will support.
I have an older iPad that can only run iOS 12.5.5 and therefore I have to deal with Numbers version 10.1.
As a result I can not open all Numbers files that I create on my MacBook with Numbers version 12.0.
Current macOS version is 12.4 and Numbers version 12.0!
Ralf