I have achieved to show item number only in the mini cart header with following changes.(OpenCart 18.104.22.168)
"Shopping Cart %s item(s) - %s"
"Shopping Cart %s"
Shopping Cart: 1
Shopping Cart: 1 item(s) - $602.00
You have to modify these two file:
First one is used for default text, and second called for events such add or delete.