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We know the Patriarchs and Matriarchs prophetically had knowledge there would be the Shevatim (12 tribes) descended through Ya'akov.
Source: (Bereshit Rabbah 72,5) concerning the fact that Ya'akov would father twelve tribes from four wives; they competed as to who would bear how many of these children for him. If the mothers knew, surely the sons knew.
Why would the brothers of Yosef conspire to kill him thereby rendering a total of 11 tribes?
One of the many explanations given of why they wanted to kill him in the first place, is that they thought he was planning to get them cursed and (thus) killed, etc. (See below link to another question with more explanations), and so they acted in self-defense. At this point, if Yosef would kill them, there wouldn't be 11 tribes, there would be only one tribe: Yosef.
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Yosef's two sons were part of the tribes. Perhaps they were thinking one of their sons would be considered a branch.
If the brothers did not legitimately think that Joseph deserved the death penalty and wanted to kill him anyway, you wouldn’t expect them to care about this either. If they did legitimately think thatJoseph deserved the death penalty then we can perhaps apply the following passage from Berachot 10a:
What did the Holy One, blessed be He, do? He brought sufferings upon Hezekiah and then said to Isaiah, Go visit the sick. For so it says, In those days was Hezekiah sick unto death. And Isaiah the prophet, son of Amoz, came to him and said unto him, Thus saith the Lord, Set thy house in order, for thou shalt die and not live etc. What is the meaning of ‘thou shalt die and not live’? Thou shalt die in this world and not live in the world to come. He said to him: Why so bad? He replied: Because you did not try to have children. He said: The reason was because I saw by the holy spirit that the children issuing from me would not be virtuous. He said to him: What have you to do with the secrets of the All-Merciful? You should have done what you were commanded, and let the Holy One, blessed be He, do that which pleases Him.
(Soncino translation, my emphasis)
Under this approach it doesn’t matter whether There are supposed to be 12 tribes or not. Man’s job is to fulfill his duties; it is God’s job to make sure everything happens in accordance with the master plan.
widower, but how do you know his other wives had already died? Also, he only had his first child at the age of 84, so you see he was fertile even at old age.