They're a technologically advanced species though. Maybe they have birth control. They might just have settled on twenty million as a good number to have. Large enough to absorb epidemics or to ensure the survival of some pockets if the humans one day decided to launch a surprise attack, small enough to not be a burden on the Earth or to make the human governments feel over-threatened.
Anyway, this was a very good episode. Capaldi's speech was very impressive. Both the delivery and the writing with little bits like "this is a scale model of war, you never know who's going to die...". I was impressed.
It did occur to me that if Zygon's went public it would revolutionize the sex industry for several interesting reasons. But I suppose Doctor Who wouldn't cover that.