Question for CMS experts / users

[WebsiteYETcom]

I am working / designing on two CMS website project ideas.
Each one involves up to 300+ domain names.
Each domain will have a unique /folder for each client.
Each domain can/will have one, and hopefully hundreds or more unique client folders.
Unique URL for each client.
DomainName.com/ClientsUniqueFolderName. (Example)

The demo CMS instructions states, the website (domain) can have just ONE page, of which the client can edit the view section, add delete pages etc.

My CMS question is ....
Can I possibly build a model page (master page so to speak), and then edit that page for each client's folder URL?
In other words, Can I build the CMS page, Then edit the script to utilize each client's /folder and upload the script to the new client's unique folder at my domain?

Something like this.
ForSale.com (example domain)
ForSale.com/johnjones (example of ONE client's /folder (URL)

Can the CMS script be made to work uniquely with one or more /folders of a domain?
A new client login name as MaryJones, can I edit the original script, for her, and upload to the folder of /MaryJones ? and same for each new client ?
Same data base, different Users / clients/ folders /logins etc.

Thank you in advance, ... I hope this is clear.