How to properly organize code on Github for multiple clients?

Question

GitHub

There is a certain CRM system (sugarcrm) that I want to fork on github. There are also about 10 clients who have this CRM installed and are already using it.
Naturally, each of the clients wants to redo something at home, for which I sometimes have to change the logic of this CRM.
From the github, I will need to freely upload code belonging to a specific client (for example, specific modules of the system will work differently for everyone), and to be able to merge the code between clients (to fix bugs common to all)
So: How will the code be organized on the github? - make several repositories for each client or start separate branches for each client, or is there any other option, tell me plz