Moving this to an "answer" of sorts for no other reason than the comments aren't long enough:
To iterate my comment:
Even though DNN supports multiple sites on one install, we never do this. "Supported" and "Works well in practice" are two very different things.
For example, different sites have different requirements which lead to different additional modules being installed. If you go to upgrade an install to the latest version you run a very real risk of breaking some of your sites. Another example is when you have to modify the core styles for one site, but not another... I really wish DNN had finer grain control over those.
Another example is the html editor. DNN provides for several different ones.. But, only one can be enabled per install. This means you pick one and stick with it for everyone. The unfortunate thing here is that, at least with DNN, there isn't one editor that clearly outshines the others as they all have issues. Other areas are similarily impacted like friendly url's and the sitemap provider. This one may be a non-issue.
With regard to hosting 1000s of sites there are many things to consider:
- If one DNN site gets hosed, it takes everything down. This might impact your SLA.
- You'll need to make sure your worker process configuration can handle the potential traffic. The default IIS config in this area is not good enough for the scale you are talking about.
- Third, it is NOT easy to split out sites to separate DNN installs if you change your mind later.
The main upside I can see for hosting that many sites in a single install is the same as it's downside. Namely around upgrading the CMS. CMS's are like any other products in that security problems and other bugs are routinely found. A properly supported CMS will issue updates as problems are found and fixed.
Updating one install is obviously preferable to updating "1000s". However, the downside is that this introduces a potentially crushing risk.
First, ALL of the sites would be down during the upgrade.. If the sites are global in nature, this might cause your customers some pain. Not ideal. Second, if the upgrade goes awry (as happens all to often with such things) then you're down for longer than anticipated simply due to having to restore from backups.
You can mitigate some of the risk by testing the upgrade procedure in a staging environment, which you should do anyway. You could also split regional sites into separate DNN installs so you are impacting only a single region at any one time.
I'd say that you really need to think this through and proceed with caution.