You export several documents from GoodNotes and choose a cloud storage app from the Share sheet. However, only one file is uploaded.
It's a known limitation of the cloud storage app. Even if GoodNotes sends multiple files, the receiving app might accept one only. Instead of choosing the cloud storage app from the Share sheet, please choose "Save to Files" and choose a location on that cloud storage. If it's not found in Files, you can add it.