Member of Immortalli within the realm of Tamriel. Explorer and weapon crafter. Restoration Ecologist
I don't know what the limitations will be in terms of numbers. Some mixed-race characters are supposed to be possible. The mixing allowed will probably follow geographic lines, so Brudvir-Janoa mixes won't be likely. You will get to choose a mixed-race character if you can find a family of mixed race with an available child slot.
From what i've been able to tell, your character starting off mixed will most likely not happen but that doesn't mean you can't make your children a mix. What the limits to that will be i'm not sure. For all we know it could be as harsh as neran not being able to breed with brudvir or hrothi or as lenient as letting brudvir breed with kypiq
Mighty gear for the Mighty Menn
All of the tribes are created from the same base model expressly for the purpose of allowing them to mix.
There is a thread talking about this in the new Tribes section of the forum.
https://chroniclesofelyria.com/forum/topic/28710/cross-genetics-in-intertribe-children