Best practice for migrating folder managed versions to git

Looking for Best practice to add 20 versions of an application into git version control it is currently managed in different folders for each version. Each version is still actively used to a subset of customers due to customer specific customization or upgrade cost. So I am bit confused that either I put each version as branch or separate repository?

We also want to apply ci/cd on each version, so that on any change in any version could be auto deploy to the customers using that version.

Please share your ideas to determine the right process?

I am using Visual studio 2017 with git hub.