Wordpress - Change Posts per page count

This will do it: (add to your theme's functions.php)

add_action( 'pre_get_posts',  'set_posts_per_page'  );
function set_posts_per_page( $query ) {

  global $wp_the_query;

  if ( ( ! is_admin() ) && ( $query === $wp_the_query ) && ( $query->is_search() ) ) {
    $query->set( 'posts_per_page', 3 );
  }
  elseif ( ( ! is_admin() ) && ( $query === $wp_the_query ) && ( $query->is_archive() ) ) {
    $query->set( 'posts_per_page', 5 );
  }
  // Etc..

  return $query;
}