How many Bitcoin forks can we expect to see?
a lot more.
so far there has been a lot of them already (about 600 or so) and there will be a lot more.
the forks of BCH type where they also use the blockchain, will also gain popularity in the near future and there will probably be a couple of hundred of those too.
How much of Bitcoin's future value will be in the main fork and how much will be in Bitcoin child coins?
the value of bitcoin will not change. it will continue as it was before, being adopted and growing.
the "child coins" will remain as altcoins, most of which useless.
there may be some sell pressure temporarily on bitcoin only if these "child coins" didn't have a way of dumping them directly to fiat. but they have had that option so far so that effect is also non-existent.
How can we make Bitcoin unforkable?
you can not.
bitcoin is open source and the license of it allows anyone to freely use that code however they wish. and it is actually a good thing that they keep forking bitcoin, it proves how strong bitcoin is and how shitty all these nonsense forks are.
I think Bitcoin will inevitably go through many forks on the trajectory it is going now. Has anybody dared to predict?
i predicted above, at least 100 more like BCH fork and like before other normal forks will be a lot more too.