How do I get the index number of an input element from Input Array

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C#

ASP.NET

Javascript

C#4.0

jQuery

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Here is my sample output on screen:

<form action="/Home/UpdateBooks" id="formBooks" method="post"><div class="isbns" style="margin:25px;">
    <p><input class="text-box single-line" name="[0].isbn" type="text" value="hlkjh" /></p>
    <p><input class="text-box single-line" name="[1].isbn" type="text" value="KhlkjhLK" /></p>
    <p><input class="text-box single-line" name="[2].isbn" type="text" value="jhlkjhL" /></p>
    <p><input class="text-box single-line" name="[3].isbn" type="text" value="saejhjkh" /></p>
    <p><input class="text-box single-line" name="[4].isbn" type="text" value="1234567890" /></p>
    <p><input class="text-box single-line" name="[5].isbn" type="text" value="1234567890" /></p>
</div>

I need to add an input element on a button click -"Add more". How do I get the last index of isbn input array so that i can increment and append to the div?

Thanks for helping in advance. I really appreciate it!