Function to limit number of custom post types on homepage – TwentyTen

This should do the trick:

add_action( 'pre_get_posts', 'add_tours_post_type_to_query' );

function add_tours_post_type_to_query( $query ) {

  if ( is_home() && $query->is_main_query() ){
    $query->set( 'post_type', array( 'tours' ) );
    $query->set( 'posts_per_page', 1 );
  }

  return $query;
}

For the future the Codex can be your friend. Exactly this is explained there.