Hello, @gaselec.
At this time the bill number limits to just 21 characters in QuickBooks Online (QBO). I understand that your bill number exceeds the limit and won't be able to enter the complete details. At this time, you can enter the 21 digits or just the characters that fit on the field provided. Then, you may enter the exact bill number on the Description or Memo section for identification purposes.
I'll also take note of this information so I can pass along the information to our product engineers. This way, they'll be able to know our customer's needs and might consider the feature in the coming updates.
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Get back to me if you have any other questions. I'm a few clicks away to help. Have a good day!