They just struggle on and readjust to the shrinking numbers as best they can until there is no one left.
One has to wonder the level of spin from Org to excuse fading numbers, especially when it becomes too obvious. They are already massaging the number in innovating ways. They might have to resort to stop counting altogether. Or maybe they'll start counting visits to the website as members of the religion. They will need to cover the bleeding somehow.
I know I've heard my mother say several times that even if there is only one person left, it will still be the true religion. Hard to beat that level of delusion.