Thousand fold world system In the Kosala Sutta (AN 10.29) it quotes the following:


  "As far, bhikkhus, as this thousandfold world system extends, Mahābrahmā there ranks as the foremost."


So basically as far as the sun and the mood illuminates the light a world system is there. 4 hells, human world , 6 humans and so forth. And thousand of this is called a thousandfold world system.

So according to the above quote, does this mean that there is only one Maha Brahma to all these thousand world systems?  Or are there 1000 Maha Brahmas in this thousand world system? Or is it just 1?
31 planes of existence coming together is one universe. In each universe there is a Maha Brahma in each Brahma plane.


  ... a thousand brahmā worlds. ...


According to the Abhidhamma there are infinite world systems. See: Abhidharmartha Pradipika By Amaradasa Rathnapala.

Also see: The Buddhist Concept of World by Bhikkhu T. Seelananda and also this answer.