Assert the number the mini cart of the header shows, or that it is empty.
Examples
Then the mini cart should show 1 item
Then the mini cart should show 2 items
And the mini cart should show 0 items
Then the mini cart should show 3 items
Then the mini cart should show 5 items
Pattern
Then(/^the mini cart should show (\d+) items?$/, …)
Part of @webship-js/magento-steps (github.com/webship/magento-steps), a step pack for Webship-js 2.0.x.