Firebase Authentication limit
If you are just concerned about the authentication, excluding phone authentication. Then here is the answer from the Firebase Pricing FAQ
all Authentication features beyond phone authentication are free.
Here is the full statement:
Which products are paid? Which are free?
Firebase's paid infrastructure products are the Realtime Database, Storage, functions, Hosting, Test Lab, and phone authentication. We offer a free tier for all of these features.
Firebase also has many free products: Analytics, Cloud Messaging, the Notifications composer, Remote Config, App Indexing, Dynamic Links, Invites, and Crash Reporting. You can use an unlimited amount of any of these products in all plans, including our free Spark plan. In addition, all Authentication features beyond phone authentication are free.