Firebase security rule trigger (got no clue here)

I am trying to increment "numberOfEvents" field in Users collection whenever an event (in Events collection) is created. How do I do this, I know the below code is wrong I just copied it from firebase documentation. Please help.

rules_version = '2';

const functions = require('firebase-functions');

function increment() {
exports.myFunction = functions.firestore
  .document('Events/{docId}')
  .onCreate((change, context) => { 
  get(/databases/$(database)/documents/Users/$(request.auth.uid)).data.numberOfEvents + 1; });
}
service cloud.firestore {

  // Do not change this
  match /databases/{database}/documents {
 
     // Path to your document
     match /Users/{docId} {
     allow read;
     allow write;
     }
    match /Events/{docId} { 
      allow read;       
      allow write: if get(/databases/$(database)/documents/Users/$(request.auth.uid)).data.numberOfEvents < 16;     
    }
  }
}