<ul id="thumbnails">
<li>
    <a href="#slide1">
        <img src="img/image-1.jpg" alt="This is caption 1">
    </a>
</li>
<li>
    <a href="#slide2">
        <img src="img/image-2.jpg"  alt="This is caption 2">
    </a>
</li>
<li>
    <a href="#slide3">
        <img src="img/image-4.jpg" alt="And this is some very long caption for slide 4.">
    </a>
</li>

// Create thumb-wrapping-elements
$('.demo_wrapper').append('<div class="thumb-box"></div>');
$('.thumb-box').append('<ul class="thumbs"></ul>');

// How many thumbs? Check how many slides there is.
var count = $("#thumbnails li").length;

// Loop through and create li-elements and links
for (var thumbcounter = 0; thumbcounter < count; thumbcounter++) {

// Whats the width of each thumb?
if (count == 1) {var thumbwidth = "100%"}
if (count == 2) {var thumbwidth = "50%"}
if (count == 3) {var thumbwidth = "33.33%"}
if (count == 4) {var thumbwidth = "25%"}
if (count == 5) {var thumbwidth = "20%"}
if (count == 6) {var thumbwidth = "16.66%"}
if (count == 7) {var thumbwidth = "14.28%"}
if (count == 8) {var thumbwidth = "12,5%"}
if (count == 9) {var thumbwidth = "11.11%"}
if (count == 10) {var thumbwidth = "10%"}

thumburl = thumbcounter + 1;
$('.thumbs').append('<li><a href="#' + thumburl + '" data-slide="' + thumburl + '" style="width:' + thumbwidth + ';">

</a></li>');

<div class="thumb-box">
            <ul class="thumbs">
                <li>
                    <a href="#1" data-slide="1">
                        <img src="img/image-1.jpg" alt="This is caption 1 <a href='#link'>Even with links!</a>">
                    </a>
                </li>
                <li>
                    <a href="#2" data-slide="2">
                        <img src="img/image-2.jpg"  alt="This is caption 2">
                    </a>
                </li>
                <li>
                    <a href="#4" data-slide="3">
                        <img src="img/image-4.jpg" alt="And this is some very long caption for slide 4.">
                    </a>
                </li>
            </ul>
        </div>

// exported and edited the thumbwidth var for efficiency
var thumbwidth = (100/count) + '%';

// Loop through and create li-elements and links
for (var thumbcounter = 0; thumbcounter < count; thumbcounter++) {


var thumburl = $('.thumbnails').find('img').eq(thumbcounter).attr('src');
$('.thumbs').append('<li><a href="#' + thumburl + '" data-slide="' + thumburl + '" style="width:' + thumbwidth + ';"></a></li>');

}