Limitation to number of documents under one Collection in firebase firestore

There is no document limit under one collection

Official documentation for clearness: Click here

But there is limitation in maximum depth of subcollections

There is no limitation on the number of documents in Firestore collection but it has a limitation of the size of the document. The maximum size of a document is roughly 1 MiB (1,048,576 bytes).

There are many more limits that are listed in the below link. Limits of firestore

firebaser here

There is no documented limit to the number of documents that can be stored in a Cloud Firestore collection. The system is designed to scale to huge data sets.