number of posts with “Load More”

You can get the (current) displaying posts number from offset number.
As it’s an ajax load more for wp post, it should has the offset numbers.
So, you can display the number like this

Javascript

var offset = 4;
return $.ajax({
        url: ajaxurl,
        type: "POST",
        data: data,
        success: function(returnData) {
        // diplay offset number in .post-number div
         $('div.post-number').html('This is the displaying post number="+ offset);
        }

 }), offset += 4;

HTML

<div class="post-number"></div>

You can add this under the loadmore div

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I am not quite sure with Ajax Load More plugin.

If you just want to customize Ajax Load More plugin, you should ask the author of that plugin.