It’s about how easy it is for you to deal with individual modules.
If you can imagine a release process that captures one module from a common repository, create a ZIP file from it, place this ZIP with it, somewhere you need composer.jsonmetadata, run this ZIP via HTTP (S), and then make Packagist (or local Satis repository you want to use) aware of this version ... then you can use one single repository.
In all other cases, one repository per Composer package is the way to go.
Please note that you can, of course, say maxinet / random set of modules as your only package. And at a later time, if you are careful enough, you can transfer parts of this module package to the same repository and re-include it in the package with the composer.
Git : Detach (move) Git