Installing and upgrading help

You have to install separate Moodle instances for each of your client.

You're title makes sense under one installation. Yes you can have many customers in one installation of Moodle. They could all be split into blocks, and from each block they can have their own brand name, create their different courses and host their own students.

But if your customers want their own moodle installed in their own server with their own domain address they can do that themselves freely by following the instructions here in moodle.org documentation 

What kind of business are you running? who are your customers; teachers, schools, universities, companies?

Examples of moodle sites providing courses from different "Brand names".

Are these examples of the kind of business you want to be? Do you want to be a moodle hosting service?

http://moonami.com/

http://www.moodlenews.com/collections/hosting/

And of course moodle partners can assist you with your business venture http://moodle.com/partners/

You can use different categories for this.  Each category can have its own theme/branding and you can set managers/administrators at the category level.