The more, the merrier!
You can add as many dogs to your Fi app as you need! See:Adding multiple dogs to the Fi App
Within your app, each dog will have their own profile with their location and activity stats. You will also receive customized alerts sent to your phone, such as "Ziggy has left home without an owner" or "Rosie has reached her step goal for the day".
To switch between each dog's profile in the app, all you have to do is tap their profile card in the menu!
Note: For remote GPS tracking, each dog will need their own collar and active Fi subscription.