Does MongoDB track how many times each index is used in a query?

Yes, you can track how many times each index is used in a query using MongoDB $indexStats. Following is the query to create an index in MongoDB −

> db.demo508.createIndex({"FirstName":1});
{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}

Let us create a collection with documents −

> db.demo508.insertOne({"FirstName":"John"});{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e883818987b6e0e9d18f578")
}
> db.demo508.insertOne({"FirstName":"Chris"});{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e88381b987b6e0e9d18f579")
}
> db.demo508.insertOne({"FirstName":"David"});{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e88381f987b6e0e9d18f57a")
}

Display all documents from a collection with the help of find() method −

> db.demo508.find();

This will produce the following output −

{ "_id" : ObjectId("5e883818987b6e0e9d18f578"), "FirstName" : "John" }
{ "_id" : ObjectId("5e88381b987b6e0e9d18f579"), "FirstName" : "Chris" }
{ "_id" : ObjectId("5e88381f987b6e0e9d18f57a"), "FirstName" : "David" }

Following is the query to track how many times each index is used in a query −

> db.demo508.aggregate([{ $indexStats: { } }]).pretty();

This will produce the following output −

{
   "name" : "_id_",
   "key" : {
      "_id" : 1
   },
   "host" : "DESKTOP-QN2RB3H:27017",
   "accesses" : {
      "ops" : NumberLong(0),
      "since" : ISODate("2020-04-04T07:32:27.394Z")
   }
}
{
   "name" : "FirstName_1",
   "key" : {
      "FirstName" : 1
   },
   "host" : "DESKTOP-QN2RB3H:27017",
   "accesses" : {
      "ops" : NumberLong(0),
      "since" : ISODate("2020-04-04T07:32:27.527Z")
   }
}