var db = firebase.firestore();

  //From the Firebase documentation

  function createCounter(ref, num_shards) {
    var batch = db.batch();

    // Initialize the counter document
    batch.set(ref, { num_shards: num_shards });

    // Initialize each shard with count=0
    for (let i = 0; i < num_shards; i++) {
      let shardRef = ref.collection('shards').doc(i.toString());
      batch.set(shardRef, { count: 0 });
    }

    // Commit the write batch
    return batch.commit();
  }

  function incrementCounter(db, ref, num_shards) {
    // Select a shard of the counter at random
    const shard_id = Math.floor(Math.random() * num_shards).toString();
    const shard_ref = ref.collection('shards').doc(shard_id);

    // Update count
    return shard_ref.update(
      'count',
      firebase.firestore.FieldValue.increment(1)
    );
  }

  function getCount(ref) {
    // Sum the count of each shard in the subcollection
    return ref
      .collection('shards')
      .get()
      .then(snapshot => {
        let total_count = 0;
        snapshot.forEach(doc => {
          total_count += doc.data().count;
        });

        return total_count;
      });
  }

  //Your code

  var ref = firebase
    .firestore()
    .collection('counters')
    .doc('1');

  var num_shards = 2  //Adapt as required, read the doc

  //Initialize the counter bay calling ONCE the createCounter() method

  createCounter(ref, num_shards);

  //Then, when you want to create a new number and a new doc you do

  incrementCounter(db, ref, num_shards)
    .then(() => {
      return getCount(ref);
    })
    .then(count => {
      console.log(count);
      //Here you get the new number form the sequence
      //And you use it to create a doc
      db.collection("events").doc(count.toString()).set({
         category: "education",
         //....
      })
    });