Here is the complete list of league names for leagues 1-20-- copied from posts by Elminster and Mike Bravard which can be found by scrolling down a lot of pages.

Leagues 1-18 are named after great baseball players. I believe they have been around for 10+ years (before purebaseball was purebaseball).

Ruth 1
Cobb 2
Speaker 3
Gehrig 4
DiMaggio 5
Musial 6
Mantle 7
Berra 8
Williams 9
Bench 10
Clemente 11
Aaron 12
Morgan 13
Banks 14
Kiner 15
Cochrane 16
Kaline 17
Foxx 18

Leagues 19-20 were-- "started as 'best of' and 'best of II' leagues but have become like the original 18 since." (Mike Bravard quote)

and they are called

Champions 19
Challengers 20