17
Reasons:
1. It's a good number
2. It's the least 2 digit number not already guessed.
3. It's conceivable that you are all underestimating (except scsn). Debt is paid off without overflow, unlike a province sometimes. Also, these 8 VP doesn't clog your deck at all.
4. It's probably supposed to be a surprising answer
5. It's a good number