Depends

Many ways to see this and probably even beyond what the originator realized.

First off we have tense and context.

Yes the tense issue says that they are moving there so no one is there yet.


Also look at the sentence context which asks how many (islands) are there which there is one.

Another contexts is the move of these people leaving behind an empty space in which the answer is zero.

Assuming that the move means they are there then there could be 2 or more depending if she is pregnant with one or more fetus.

A metaphysical way to interpret this is that there is only one there due to the omnipresence of all that is or you could say everyone is there (hundredth monkey).

On and on....