Reliably tracking users with 100% accuracy without requiring the user to sign in is next to impossible. It’s somewhat possible using browser agent info, or attempting to collect user info without consent, but not 100% reliable. All major browsers make it very difficult to do. It’s a battle that ad agencies attempt to overcome every day, which also forces browsers to lock things down more to protect user privacy. I should add that attempting to do something like this opens up a whole bunch of security holes because none of the data will be secure in a public app.
Also note that you can keep your app public and make certain parts available only when a user signs in. This allows for positive identification of an individual and allows for proper data security if you use things like rows owners and/or roles.
Curious how you came up with this number. The only plan that allows “unlimited” users is the Maker plan, although you are restricted to users with personal email domains only, and you don’t get all of the features of a higher plan. Something like the Business plan only gives you 30 users, and then you are billed $5 or $6 per additional user. The Enterprise plan is negotiable on the users.