ok, got it !
$args = array(
'post_type' => 'maison-hotes',
'post__not_in' => $post_id,
'meta_query' => array(
'relation' => 'OR',
)
);
foreach($cities as $cities_id) {
$args['meta_query'][] = array(
'key' => 'contact_city', // name of custom field
'value' => '"'.$cities_id.'"', // matches exactly "123", not just 123. This prevents a match for "1234"
'compare' => 'LIKE'
);
}