Comment by mamcx

Yeah, this is the way for the small/mid tier (this is also how I do for https://www.bestsellerapp.net)

What make this complicated is that each company turns into a different project. So is like have several branches:

    ERP
       /CompanyA
       /CompanyB

This is high maintenance the more successful you are (and puts a serious barrier to adapting to certain customers).

It gets to a point where after a certain # of customers you can't grow, and now is where you think about making your own DB, programming language, ... :)