Add these few lines of code to your theme's functions.php file to allow you to change the number of products per page

functions.php

add_filter( 'loop_shop_per_page', 'new_loop_shop_per_page', 20 );

function new_loop_shop_per_page( $cols ) {
  // $cols contains the current number of products per page based on the value stored on Options -> Reading
  // Return the number of products you wanna show per page.
  $cols = 9;
  return $cols;
}