Oda will go with Motive of dream.
Her dream directly put her against Nami.
Someone has to stop Imu , if Nami wants to complete her dream.
Maybe that was the reason we were reminded about Nami's dream in cover page.
First part of her dream is seeing everything with her own eyes.
Making map of the world is second priority.
Although she may actually draw world map in eos (shorter verison).
But that is not gonna end her "eternal dream " but that will be good conclusion to it.
As Marco said there is almost 10 or 20 million. He basically means countless.
Minks think there were only 5 islands, I think they were talking about continents.
In past maybe there used to be only 5 continents.
Maybe somehow at the end of one piece everything will go back to what it used to be before void century. (I hope Nami is the one who will gain the power to bring back these islands back )
But that isn't gonna fulfill Nami's first dream but atleast it will be satisfying conclusion for the story.
Of course nothing stopping her to continue her journey even after one piece ends..