The order quantity is certainly stored in the [TotalQuantity] field of the [OrderProduct] table in the uStore SQL DB.
I would try creating a Dial Variable in uPlan, then a related hidden Dial in uStore, bound to a SQL query (with the "Override existing value (may override a value selected by the shopper and in reordering)" and "Take Values from Data Source" options).
Now, I do not know, what you want to do with this information, but as the "Quantity" will certainly be specified by the uStore User in the OrderFinalStep page only, I would not hope that the value in uPlan is accurate, until the User reaches this step.
In other words: the value should be the wrong one in the Customization Wizard, as it is located *before* the OrderFinalStep in the user journey.