How to use WP_Query to display many posts?

The p (post) paremeter for WP_Query accetps a single value only:

$args = array(
    'p' => 100
);

For multiple posts use post__in and pass an array of post ids:

$args = array(
    'post__in' => array(100, 102, 105)
);

See Post & Page Paremters for further clarification.