Even though this is answered, stating some useful information and facts.
The technique of Trigrams is to hit as many chakra points as possible consecutively (in powers of two, basically hit 2 times consecutively and then hit another 2, to make it 4 consecutive and so on). As you have told already, anime has shown up to 128 chakra points, whereas a human in the Naruto series has 361 Chakra points (Source).
Therefore it can go up to 256 palms because, after 256 palms, someone can't hit another 256 palms, making it 512.
And it has been stated multiple times that even hitting up to 64 palms can render an opponent unable to stand, therefore 256 may affect even the vitals of the opponents most likely killing them in turn (Source).
As the technique requires both speed and stamina along with precision, apparently no one throughout the series could reach 256 palms, who had enough speed, stamina, and precision for it. The most gifted Hyuga, Neji achieved the feat of 128 palms, which is the highest in the series, and probably in Hyuga history as well.