in DotNetNuke, an installation can host as many portals (parent or child) as technically possible - there are people with thousands of it. please notice, that the portals do not share data or files, only the plattform and modules and occasionally skins.

If you have your DNN installation, you can add a portal by clicking add in host :: portals menu item. Please notice, that the web server needs to assign the domain name (in your example parentSubDomain.mywebsite.com) to the DNN root directory.

A DNN installation (with single or multiple portals created inside) uses a single database and the same application files imcluding modules, language packs and shared skins. Within subfolder "Portals" , each portal has a folder to store portal specific files.

In your name server and IIS need to point all relevant parent portals pointing to the DNN web. For a domain, you can use a wild card, to direct all servers (as default www) to the same web site. This has to happen in name server and IIS outside DNN.