get the selected options and validate the number of marked options?

Ionic Framework ionic-v3
1

I’m using an ion-select and I’m enabling the multiple attribute to select several options. I can not find a way to disable the rest of the options in real time if 3 options have already been checked. I am currently using the ionSelect event, but it only works when an option is checked. How can I solve my problem? How can I solve my problem? I would like to understand how I know how many options I have marked and get their value in real time.

this is my code: https://stackblitz.com/edit/ionic-8wewvd?file=pages/home/home.ts

pages/home

<ion-label>Select a person</ion-label>
<ion-select [(ngModel)]="person" multiple="true">
  <ion-option *ngFor="let item of options; let i = index" 
    [value]="item.id" (ionSelect)="fn_checkOptions()" >
    {{item.name}}
  </ion-option>
</ion-select>

export class HomePage {
public options:object=[];


constructor(public navCtrl: NavController) {
this.options= 
[
  {"name":"pedro","id":1},
  {"name":"juan","id":2},
  {"name":"maria","id":3},
  {"name":"velez","id":4},
  {"name":"yaya","id":4}
 ]
}

 fn_checkOptions(){
  alert("hey")
 }

}