How can I track on Analytics which sites installed my WP plugin?
Analytics cannot determine which websites have installed your plugin. You will probably have to hardcode that into your plugin.
If your plugin is an API with data hosted on your website then maybe you can track the websites which are using the website as referral websites.
You can although track the number of downloads, number of page visits for the plugin. On the other hand, if it is a premium plugin, you can ask people to fill in a form to let you know which website they will be using the plugin on.
Hope this helps.