Right now, you can only scan 21 receipts on a rolling 7-day period on the app. This means you can keep scanning if your oldest receipt falls outside of this 7-day period.
To further explain this, an example would be:
If you scan 19 receipts on Wednesday, and 2 receipts on Thursday, you would have to wait until the next week Wednesday for those 19 scan slots to open up again, and until Thursday for the other two to open.