wp-ecommerce plugin short code not working

I was trying out with the shortcode category using:
[wpsc_products category_id=’4′]

however when I try to set only 2 items per page:
[wpsc_products category_url_name=’test-category’ number_per_page=’2′]

It doesn’t work? I have 3 items in that category and after setting to number per page = 2, it still display all my 3 items?

beside what is the difference between limit_of_items and number_per_page??

Thanks for the help.