No more than 8 survivors will ever be alive, outside the saferoom. Once there are 8 survivors, no more will appear until some of them die or reach the saferoom. I think there may sometime be exceptions to this and some survivors upto the total of 8 will appear. If you have 6 survivors and the next to appear are a group of 3 or more then, they won't usually appear. I remember encountering this problem, I had 7 survivors and the next to appear were a group of 2, they never did so, I had to replan which 8 survivors I was going to rescue. I don't recall exactly what group I went for but, I'm sure it involved the 5 girls from above the law.
The tougher the battle, the sweeter the victory!!