Changing posts per page on the fly From http://wordpress.stackexchange.com/questions/30757/change-posts-per-page-count/30763#30763

change-posts-per-page-on-the-fly.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;
}