You can add unlimited websites to track in Piwik, in the Admin Websites section. In fact, Piwik scales well to thousands of websites, and a few Piwik users are tracking even more. Each Website is defined by a URL. You can track sub domains in the same Piwik website, or you can create a different website for some of your subdomains. Learn more about creating Websites in the Managing websites page.
The ‘All Websites’ dashboard is a useful first page to look at for a quick overview of the performance of all your websites over the last day or week. You can even make it your default page to load in Piwik (go to top menu Username > User settings). If you track more than 10 websites, the Website selector will display the first ten sites only, and show a search field where you can search quickly to look at another website.
In the Admin Websites page, you can also define for each website a list of Alias URLs which contains all the other domains and subdomains that will be tracked. This will ensure they don’t appear in your “Referrer” reports.
If you track many subdomains in the same Piwik website, you might want to see the domain name directly in your Page Title reports. If you want to show “support.example.org/Homepage” instead of simply “Homepage”, check out the JS setDocumentTitle documentation.
To measure how many users have visited two or more of your websites on a particular day, use the InterSites plugin from the Marketplace.
The official Piwik Mobile App supports all these reports, including the ‘All Websites’ dashboard.