Display number of options selected as soon as they are selected using a multiple choice select2

Question

So, I have this multiple choice select2 that gets its options via database query and I wanted to show in an input the number of options that are selected. However, I wanted to do it as soon as they are selected and I don't know how to do it. For example, if I choose 2 options and then, later, I decide to add one more, I want the input to change to 3 options selected. Any help is appreciated and if you have other ideas like how I can do similar stuff but not like how I described it your help is appreciated too. Thanks

<select name="select_areas_educ_form" id="select_areas_educ_form" class="form-control js-example-tokenizer" multiple="multiple">
<?php foreach ($areas_educacao as $area)
{
    echo '<option value="'.$area['id_areaeducacao'].'">'.$area['cod'].' - '.$area['designacao'].'</option>';
}
?>
</select>

Answer 1

So, its much more simple than what I expected, so if anyone need the answer I'll leave it here. You just basically need to add an "onChange" in your select that calls a function. The function will count the number of options selected via jquery and after, change the placeholder of the input. Its very simple and now I can't explain how I didn't think of this before

function opcoes_select_areas_educ_form()
  {
    var n_opcoes = $("#select_areas_educ_form :selected").length;
    document.getElementById("total_areas").placeholder = n_opcoes;
  }