I want to create a segment which gives the number of unique cart opens using cart events. There should be atleast one cart addition and order may or may not exist
In case you've implemented "scopen" you can leverage the same to create your segment. The Carts metric is incremented by one, whenever you fire the scOpen event. The majority of commerce clients populate the Carts event upon the initial creation of a shopping cart. Commerce solutions like Adobe Commerce, previously known as Magento Commerce or Shopify typically create a cart once the visitors add their first product to it:
By "unique cart opens" I assume you mean per visitor or per visit...
Either way, in Workspace while you can't stack metrics, you can create a segment for "Cart Opens exists" then stack that segment with your UV or V metrics to get "unique"