How to check with Rules if a new element has been added to a list in a node?
You're counting the wrong thing...item data is a bit further down the rabbit hole (inside the language array):

$count_new = count($node-&gt;field_lista_gestori[$node-&gt;language]);
$count_orig = count($node_unchanged-&gt;field_lista_gestori[$node-&gt;language]);