I have seen it depicted as one main island with a few little islands surrounding it before.
Someone here said that Themyscira is the capital city?
I looked it up just now and this is what it says, though I haven't heard it referred too much as "The Paradise Islands":
https://en.wikipedia.org/wiki/Themyscira_(DC_Comics)
I think in the movie, the main island is called both Themyscira and Paradise Island. There might be some smaller islands surrounding it, I'm not sure.
In the New 52 comics, to replenish their numbers, they show the Amazons boarding ships, making the men on board impregnate them, killing the men and then when they give birth, if they give birth to boys, they sell them off to Hephaestus and keep the girls.
I don't think the movie will go that far. I guess what they could do is ask the gods?
Because they are immortal, they're usually depicted as the same set number of Amazons.