According to Official Guinness Records,
The smallest affiliated football league is The Isles of Scilly Football League (UK) which has two registered teams. The league is played in the Isles of Scilly, UK. The league was established in the 1920's and currently involves the Garrison Gunners and the Woolpack Wanderers, who play each other 17 times over the course of the season. They also compete in two cup competitions which are played over two legs.
For a complete list of 2016 records, please visit 2016 Guinness Records in United Kingdom.