This gets asked a lot, we use WP core masonry so we can use its built in events. This example fires AFTER you click load more and AFTER masonry has added new posts
(function($){
$(document).ready(function(){
$('.fl-builder-pagination-load-more .fl-button').on( 'click', function(){
$('.fl-post-grid').one( 'layoutComplete', function( event, items ) {
console.log(event)
console.log(`Appended ${items.length} items`);
});
});
});
})(jQuery);