You can add unlimited websites to track in Matomo (Piwik), in the Admin Websites section. In fact, Matomo scales well to thousands of websites, and a few Matomo users are tracking even more. Each Website is defined by a URL. You can track sub domains in the same Matomo 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 Matomo (Piwik) (go to top menu Administration > Personal > 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.
When you want to measure multiple website domain names into the same Matomo (Piwik) website, you need to setup Cross Domain linking in Matomo to ensure that your visitors will be accurately tracked across domains into the same visit. Learn how to setup cross-domain tracking in our FAQ How do I accurately measure a same visitor across multiple domain names?
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 Matomo (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.
The official Matomo Mobile App supports all these reports, including the ‘All Websites’ dashboard.