In order to change the limit to the products you should make modifications to also_purchased_products.php, this file is located under the modules directory.

However, before making any changes, remember to first create a directory under modules with your template name and the file.

include/modules/YOUR_TEMPLATE/also_purchased_products.php

Open the file and make the changes to around line 25

Code:
 // show only when 1 or more and equal to or greater than minimum set in admin
  if ($num_products_ordered >= MIN_DISPLAY_ALSO_PURCHASED && $num_products_ordered > 1) {
Change the >1 to meet your needs...