If I did the math right, the odds of getting 17 or more tributes is roughly 1 in 310,000
For 18 or higher it jumps to 1 in 2.1 million. Or roughly 3000 people collection 12 tributes a day for 2 months straight.
edit: forget the slight boost from guild bonus, changing from 20% to 21.2%. Numbers changed to reflect this.