2 issues
1) you need to supply the post ID for both getting and setting a field.
2) count may be giving an error if no value is being returned
function acf_save_post_nombre_avis ($post_id) {
$nombre_avis = 0;
$avis_clients= get_field('avis_clients', $post_id);
if (!empty($avis_clients)) {
$nombre_avis = count($avis);
}
update_field('nombre_avis', $nombre_avis, $post_id);
}
add_action('acf/save_post', 'acf_save_post_nombre_avis', 20);