What is the number of fraudulent votes required to make it a serious threat? Hundreds? Thousands? How would you know a priori if it is a serious threat without

What is the number of fraudulent votes required to make it a serious threat? Hundreds? Thousands? How would you know a priori if it is a serious threat without the final tally? Gore v Bush 2000 had a very close total in Florida. Would it not be a serious threat in a similar situation?