Due to the nature of uStore (asynchronously subcontracting compositions to uProduce, only once the order is processed in the back office, or via automated Triggers), I indeed doubt that this information can known, before the order is placed - AFAYK, even the uProduce API only gives you the number of processed records, but not necessarily the number of pages.
However, maybe you could consider this terrible workaround, if you and your application are patient enough:
Production.GetProductionOutputUrl
gives you the URL of the produced file.
Then, a PDF library may help you count the number of pages.
Terrible, as I said, because the availability of this URL may be very asynchronous (when the order is on hold in the Back Office, on purpose), but also because the produced file may be heavy.
I hope that other members will have a better idea, I'm not proud of mine.