Hi,
Sorry this is the proper code, now the 2 lines are not the same.
Open the following file:
/YOUR_ADMIN/includes/modules/product/collect_info.php
Look for the following line:
<td class="main"><?php echo zen_draw_separator('pixel_trans.gif', '24', '15') . ' ' . zen_draw_input_field('products_quantity', $pInfo->products_quantity); ?></td>
Replace the line with the following
<td class="main"><?php echo zen_draw_separator('pixel_trans.gif', '24', '15') . ' ' . zen_draw_input_field('products_quantity', ($pInfo->products_quantity?$pInfo->products_quantity:5)); ?></td>
I have set the default quantity to 5, you can change the number to a different one.