To get the id of all posts that have been trashed when selecting multiples, you must use the

$_GET['post']

Like that example below:

function wpdocs_trash_multiple_posts( $post_id = '' ) {
    // Verify if is trashing multiple posts
    if ( isset( $_GET['post'] ) && is_array( $_GET['post'] ) ) {
        foreach ( $_GET['post'] as $post_id ) {
            wpdocs_your_function( $post_id );
        }
    } else {
        wpdocs_your_function( $post_id );
    }
}
add_action( 'wp_trash_post', 'wpdocs_trash_multiple_posts' );