What if there are an odd number of twists in a twisted pair cable?
A even number of twists is better, but I am not aware of practical cable situations where this is worth the trouble: there are other sources of interference which are probably more important that the small difference it would make.

Another way to look at it: the amount of magnetic interference is proportional to the area between the two wires. With a perfect even number of twists the area is effectively zero. With an odd number of twists it is essentially one twist area. That is still a vast improvement over no twist at all :)