how to manage branches of a project for several clients using git or bitbucket

I believe you understood already by reading the title of this post. For more explanation, Let say, I have an ERP System with both Server Side (Back end) and Client Side (Front end). Now If I have more than 20/30 clients of that ERP System, what will be the best approach to manage my code branches?

Note: there might have many scenario. for example:

1. 10 clients out of 20 clients would have same code and same settings.
2. 10 clients out of 20 clients would have same code but different settings.
3. 10 clients out of 20 clients would have different code and different settings

Here we obviously can use

1. Dev Branch
2. QA Branch
3. UAT Branch
4. Prod Branch

But what would the best approach to handle all 30 or more clients feature changes? I will be glad if you shared your experience here.