Matomo (Piwik) lets you measure several websites within one Matomo server. Each website added into Matomo has its very own ID Site (or Website ID). To find out the idSite value for a given website, log into your Matomo, then go to Administration (click on the gear icon in the top right of the screen), and go to the Measurables > Manage page (it might be called Websites > Manage. In this page you will find the list of all websites. The website ID is on the left of this table listing all websites (below the website name).

If you are using Matomo for WordPress, your Site ID should be 1 and you can double check this id in Matomo Analytics > System Report > Matomo Blog idSite.