Get the total number of elements in PHP array of textbox (inputs)

This situation basically happens when we create an Array of elements that get submitted using PHP. And the input values get retrieved using $_POST variable in array format. I made an interface where I created HTML form with one textbox as telephone_no with a button Add more. When I click this button, it generates a new textbox element with the same name telephone_no. I wanted to submit them as PHP array variables, so I write the textbox element name as telephone_no[].  So the textbox elements in HTML code look like this :

<input type="text" name="telephone_no[]" id="telephone_no" /> 
<input type="text" name="telephone_no[]" id="telephone_no" /> 
<input type="text" name="telephone_no[]" id="telephone_no" /> 
<input type="text" name="telephone_no[]" id="telephone_no" /> 
<input type="text" name="telephone_no[]" id="telephone_no" />

On the backend, I made a PHP script to catch all these telephone numbers using $_POST[‘telephone_no’] array variable. But on front-end, I also wanted to calculate how many telephone numbers were added, which can be done using jQuery.  But if you do it like below, it returns only 1 in place of 5 elements.

alert($('#telephone_no').length);

So I used the below code which worked perfectly.

alert($('input[name="telephone_no[]"]').length);