I'm pretty sure it's 20, plus the normal spawning chicken lizard, and the designer was pretty clear too, Sakkarah's comment seems pretty definitive ;
http://vboards.stratics.com/uo-tame...e-chicken-threads-stats-pics.html#post1499676
"There's a total of 20 different hues for the battle chicken."
Now since it's UO, there might maybe be a slight chance of another having been included accidentally, although I've never noticed one through all those I've raised, traded or watched in the League fights. I'm afraid I suspect Jack's eyesight is doing what mine does regularly, and two of the closer shades are misleading him - easily done when the background colour they are standing against is not the consistent when you look at the a few over time...
Otherwise though, the only way to confirm is probably to give Jack a rather fiddly job of making and posting pictures of his bunch, all against the same neutral colour background, and seeing what the results are.