Is firebase OTP is free or there is a usage fee?

Per the Firebase pricing page, you have free 10K successful verifications every month. In a month if you cross more than 10K verifications, then you might be charged.

Prices are per successful verification.

On the Blaze plan, Phone Authentication provides a perpetual free tier. The first 10K verifications are provided for free each month. You are only charged on usage past this free allotment.

All other Firebase Auth features are free to use on all plans.

Reference : https://firebase.google.com/pricing/

To check the prices of Firebase products, please check the link below:

https://firebase.google.com/pricing/

According to the link:

Free Products:

Authentication (except Phone Auth), Analytics, Predictions, App Indexing, Dynamic Links, Invites, Remote Config, Cloud Messaging (FCM), Performance Monitoring, Crash Reporting, and Crashlytics.