I made an official count of all returning castaways based on the season they first appeared in. Breaks were put in between for the all-returnee seasons.
Borneo: 6
Australian Outback: 8
Africa: 3
Marquesas: 2
Thailand: 1
Amazon: 2
Pearl Islands: 4
------------------- Allstars TOTAL FOR 7 SEASONS LEADING UP TO IT: 26
Vanuatu: 2
Palau: 3
Guatemala: 0
Panama: 4
Cook Islands: 4
Fiji: 1
China: 4
Micronesia: 1
Gabon: 3
Tocantins: 4
Samoa: 4
------------------- Heroes vs. Villains TOTAL FOR 11 SEASONS LEADING UP TO IT: 30
Nicaragua: 1
Redemption Island: 3
South Pacific: 3
One World: 4
Philippines: 2
Caramoan: 0
Blood vs. Water: 3
Cagayan: 6
San Juan Del Sur: 3
Worlds Apart: 4
---------------------- Cambodia: Second Chance TOTAL FOR 10 SEASONS LEADING UP TO IT: 29
Kaoh Rong: 4
Millennials vs. Gen X: 2
-------------------------- Game Changers TOTAL FOR 2 SEASONS LEADING UP TO IT: 6
Healers vs. Heroes vs. Hustlers: N/A
Ghost Island: N/A
David vs. Goliath: N/A
Guatemala and Caramoan have not produced any returnees. Australian Outback has produced the most overall, but Cagayan is the most modern season to come close. Cagayan is tied with Borneo when it comes to number of returnees. The longest span of non-all-returnee seasons was between Allstars and HvV. The shortest took place between Cambodia and Game Changers.
