the code for showing unique items in the cart is:
UBound(eCart1.Items)
you could create a message showing how many more items they can add using something like:
<% If (UBound(eCart1.Items) > 0) Then %>Yow have <%= UBound(eCart1.Items) %> items in the cart. You can add <%= (23 - cInt(UBound(eCart1.Items)))%> more.<% End If %>