How many users can simultaneously listen to changes in one document in Firestore or Realtime Database?

Question

In Firestore or Realtime Database, how many active users can listen for changes to the same document simultaneously? For instance, if 1000 users are subscribed to updates on one dokument will all users reliably receive notifications when that field is updated? Are there any limits to the number of listeners per document or per field, especially in terms of scalability?

  const documentRef = doc(fireStore, 'app-version', 'GDqbs283dl4lwOncr6WT');
    const unsubscribe = onSnapshot(
      documentRef,
      (docSnapshot) => {
        const data = docSnapshot.data() as { appVersion: string } | undefined;

        if (data?.appVersion) {
          setLocalAppVersion(appVersion);

       //...
       // if the app's local ver is diffrent then we have a new update
        }
      },
      (error) => {
        errorHandling(error, 'useSessionHandler onSnapshot');
      }
    );
  }, []);
  
  
  // Rules 
   match /app-version/{documentId} {
      allow read: if allowIfAuthPremium(request,userId) // only premium users are illigible.
      allow write: if false; // Deny all write operations
 }

Answer 1

Firebase's Realtime Database allows up to 200,000 simultaneous connections per database instance. See the documentation on Realtime Database limits for this and much more info.

Firestore has no documented limit. There is of course a physical limit, but it's much (seriously: much) higher than 1,000. See the documentation on Firestore limits and quotas for actual documented limits.

subscribed to updates on a single field within a document

There is no way to subscribe to only updates to a single field in a document in Firestore.