SO example

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  isFullView:false,
  model:[{name:'a'},{name:'b'},{name:'c'},{name:'d'}],
  actions:{
    seeMore(){
      this.set('isFullView', true);
    }
  }
});

application.template.hbs

{{#if isFullView}}
  {{#each model as |item|}}
    {{item.name}} <br> 
  {{/each}}
{{else}}    
  {{#each model as |item index|}}
    {{#if (check-index index 2)}}
      {{item.name}}<br>
    {{/if}}  
  {{/each}}
    <button {{action 'seeMore'}}>See more...</button>
{{/if}} 

helpers.check-index.js

import Ember from 'ember';

export function checkIndex(params/*, hash*/) {
  const leftValue = params[0];
  const rightValue = params[1];
  return leftValue < rightValue;
}

export default Ember.Helper.helper(checkIndex);

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}