firebaser here
The first line on the main documentation page for Cloud Firestore:
Use our flexible, scalable NoSQL cloud database to store and sync data for client- and server-side development.
A bit lower in key capabilities:
Designed to scale
Cloud Firestore brings you the best of Google Cloud Platform's powerful infrastructure: automatic multi-region data replication, strong consistency guarantees, atomic batch operations, and real transaction support. We've designed Cloud Firestore to handle the toughest database workloads from the world's biggest apps.
That tells you that the folks that created Cloud Firestore thinks that scalability is one of its key features. Give that these are the same folks that worked on many of Google Cloud Platform's features, I tend to believe them on that front. :-)
There were also some earlier questions that may be relevant:
Designed to Scale -- Cloud Firestore will be able to scale better than the Realtime Database. It's important to note that your queries scale to the size of your result set, not your data set. So searching will remain fast no matter how large your data set might become.
Just keep in mind that Cloud Firestore is still in beta and will be slower (but scale better) than Firebase's realtime database: