Hi,
I make some code for your requirement.
check this once
place this code whereever you want like in datacontrol or in another control
<div id="addmoreid">
<input type ="checkbox" onclick="f1()" />
</div>
<div id="addmorenum"></div>
And javascript contains following code
function f1() {
document.getElementById("addmorenum").innerHTML = "you selected count is <div id="\"cnt\"">1</div>";
document.getElementById("addmoreid").innerHTML = "<a href="\"#\"" onclick="\"f2()\"">+</a>";
}
function f2() {
document.getElementById("cnt").innerText = parseInt(document.getElementById("cnt").innerText) + 1;
}
Here you can use Jquery to post data to server when user clicks on + button
first check this functionality then you can implement this to achieve your requirement.
All the Best