Board Thread:Manga/@comment-44423638-20191116092718/@comment-1327106-20191117002836

Razzylada wrote:

If, in more than 1800 years, the number of shifters remained at nine, then it means the powers can't be divided anymore : if it was possible, then we would have more than nine shifters for sure. In 1700 years, there's no way they haven't tried to divide some shifter powers to get more, especially has we know that they already have done many experiments, some of them giving us the Ackermans.

This is likely the reason. We know the Eldian noble houses had a lot of in-fighting, so somebody would eventually consider that two Titans is better than one.

Someone else also suggested that that Sheena, Maria, and Rose were not the only ones who ate from Ymir, and that six of the other onlookers might have as well. It's an idea with little in-universe support, but if that had happened, it may well be that there are only nine Titans because there were only nine original eaters.

(The reason for the suggestion of six unrelated people is that there are Subjects of Ymir who are not of royal blood, so the non-royal Titans could not have descended from Ymir's daughters.)