Migration of 300+ partners with 15.000+ employees

We have decided to migrate to Captivate Prime. Currently our database covers 300+ partners (resellers) with over 15,000 employees assigned to them. The bigger issues I wanted to discuss is the best way to structure them in CP. I know the concept of internal and external users but I have no idea how the mechanics of this different user types are and what does it mean later on for administration. In our database each reseller has one or more managers who are able to do some reporting, and we for sure want the same in CP - honestly we are just in front of a big question which we can not solve. Any help welcome.