New code mixed into yours and commented
<?php
$images = get_field('photo_gallery','17');
if( $images ):
shuffle($images); // randomizes the image array
$max = 10; // set the max here;
$count = 0; // current count
?>
<ul>
<?php
foreach( $images as $image ):
$count++; // increment count
// if count is greater than limt, stop showing images
if ($count > $max) {
break;
}
?>
<li>
<a href="<?php echo $image['url']; ?>" rel="lightbox">
<img src="<?php echo $image['sizes']['photo-gallery']; ?>" alt="<?php echo $image['alt']; ?>" />
</a>
</li>
<?php endforeach; ?>
<div id="clearfloat"></div>
</ul>
<?php endif; ?>
