Can't find an answer, but 6 would make sense to me. Only 3 duchies are needed for a new kingdom which is 25% of the starting total per kingdom, 6 would meet this same ratio

We do not have an answer to the question at this time.

They moved away from a few of previous thoughts on the independence mechanisms, and domains have a very large range of size and population so a flat number of domain would probably not be any good way to look at it.

What we know and, to my knowledge, has not changed, is the desire to not see the creation of domains unable to provide the environment SBS deem necessary to a pleasant gameplay.

The very definition of this concept has most likely evolved significantly since early designs and will probably keep evolving as a design and later into the game translation of it.

What will be provided by roughly 1/4th of a kingdom at launch might very well be providable by a smaller group later into the game, so the actual numbers might change.

there is also the option for them to go the way they went with merged domains and let us go all the way to the bare minimum they deem necessary or maybe even go as far as what happened with merged settlements and let all hell loose..... only time will tell.

I agree with markof, the size of counties varies wildly and on top of that there are some multi-counties made up of several other counties (that now count as 1 county, even though they used to be 6 counties).

So it may be more difficult than just a number of counties, but maybe a combination of number of cities within the county, county size, maybe even county (or count) fame.

Then there's also more unknowns like the adventure islands (will they be able to declare independence, for example) and there's already some conflict within kingdoms (in Selene for example there's a kingdom that wants to cannibalize a neighboring kingdom, while in Luna a kingdom has split in half, unofficially at least). So, we will see, only time will tell.