If you are using a Repeater to show products, you can use the "TopN" property to limit the number of items shown.
For a shopping cart, i am not sure i follow you, why would you only want to show 1 item in the shopping cart?
Theoretically if you needed to, you can also use a Repeater with Custom Query to look up the current user's cart and grab 1 item out of it, or use Kentico's macro engine which has through the EcommerceContext the current user's cart and it's content that you can then reference an "ApplyTransformation" to.