title: How to limit documents in a collection in Firebase Firestore
body: <p>My users can create documents (let's say tasks) in a subcollection with a bunch of security rules checking for authentication, permissions and data validity. They can even select multiple tasks and copy them in the same collection.
  Now, a regular user will likely create at most a hundred tasks at once, but what if someone with bad intentions manage to obtain my database credentials, authenticate and try to create a huge number of valid documents programmatically? This will result in Firestore scaling without problems and an unexpected surprise in my Firebase billing.
  This is my first concern, but I'm also thinking about the possibility to limit a collection size for other reasons, and it would be at the same time a solution for the problem described.</p>
  
  <p>I read about techniques to count documents in a collection described in the Firestore documentation, but I did not found a solution.
  Keeping a counter on a doc field updated with a transaction in a cloud function would be inefficient in my case. Distributed counters increase the complexity of my data model a bit, and also I would not know how to properly read those counters in security rules for every task creation, and even if that would be an efficient solution.</p>
  
  <p>Does anyone has suggestions?</p>