var app = angular.module('plunker', ['ngTagsInput']);

app.config(function(tagsInputConfigProvider) {
  tagsInputConfigProvider
    .setActiveInterpolation('tagsInput', { placeholder: true });
});

app.controller('MainCtrl', function($scope) {
  var maxTags = 5;
  
  $scope.$watch('tags.length', function(value) {
    if (value < maxTags)
      $scope.placeholder = 'Add ' + (maxTags - value) + ' tags';
    else
      $scope.placeholder = 'No more tags!';
  });	
});
<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="http://mbenford.github.io/ngTagsInput/css/ng-tags-input.min.css" />
    <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.4.2/angular.min.js"></script>
    <script src="http://mbenford.github.io/ngTagsInput/js/ng-tags-input.min.js"></script>
    <script src="app.js"></script>
  </head>

  <body ng-controller="MainCtrl">
    <tags-input ng-model="tags" placeholder="{{placeholder}}"></tags-input>
    <p>Model: {{tags}}</p>
  </body>

</html>