Politics (in addition to other answers):
Usually, when people suddenly spawn in your country, you have to put them somewhere (at least until you can kick them out or decide to keep them). In this case, the first thing your average political leader will try to do is to send them all back home. Once it has been realized that it can't be done (because magic), then the fun will start.
So let's assume that all succubi escaped from many portal scattered around the world, you'll have 27% more human-like creatures on earth. To scale it down to a country, It means 86.1 million illegal immigrants in the US you'd have to
throw over a wall house and feed.
Any average, normally, constituted political leader would freak out without a doubt and send the military after you succubus horde because they are demons, and it is somewhat accepted in the western culture that demons have to die (because of Christian roots). No matter how tough they are, a lot will indeed die in the first few months, probably a lot of humans too.
I'm not telling that it might end up in a war, I'm telling that it cannot NOT end up in a war, 2 billion is too much, there is no good political answer to this number of 2 legged sentient things, you can't house them, you can't feed them, heck you can barely stash them.
So since I know it's probably not the story you want to tell, here are two suggestions:
- Reduce their number to less than 300 million (around 4% of world population). it's still a heck of a lot but it's bellow the bar of "oh shit we gotta do all we can now" and even if a witch hunt will start, it'll not be an open war between demons and mankind, because there are not enough enemies.
- Instead of making them pop-out out of thin air, turn 2 billion female humans into succubi (maybe through permanent possession). In this case open war in unlikely because it'll be very easy for succubi to hide behind their host's former identity and very hard to go on an open war against them. There'll still probably be some form of witch hunt performed by the authority just like in the previous scenario.
In a witch hunt scenario, you'll get a lot more interactions between succubi and humans that you would in an open war scenario. In this case, everything suggested in other answers apply and you can put the witch hunt in the background of your story far more easily that you can with an open war