Depends on if they count the Ocean biome as a true biome or not in their count. If they don't count the Ocean (I personally wouldn't... considering it has no real content) there is one more Biome after the Far North.

But just because I wouldn't count the Ocean as its own Biome, doesn't mean they don't, it is technically a Biome, with one(1) unique mob.

The only other Biome I can think of would be the World Tree itself, Yggradasil There's no room on the map for anything else.