Our brain is composed of neurons. Different subsets seem to have more or less specialised functions. Any subset can be seen as it own 'thinker'. We think of them as larger units based mostly on the amount of communication / how independently they can function.
Sometimes it is helpful to think of our mind as been made of many smaller 'thinkers'. Though there is no crisp boundary, every neuron is its own individual, at one extreme.
And I think it is sensible to speak of groupthink as a mind, with thinkers contributed by lots of people. So at the other extreme all is one super mind.
I think it can be useful to think of 'god' that way (as in the will of god, the desires of god, the thoughts of god), as much as a corporation, a culture, a nation. Though I admit you have to take care not to get carried away with the implications and end up mired in sloppy reasoning.
It is also not surprising that, because axon connections are many orders of magnitude more efficient than language, the most useful grouping is at the level of an individual.