I would come to the number by calculating how people will support themselves and others, and yet have a reasonable load on them.
That number seems to be at least greater than 3. So 4. Probably it's more like 3.1 or so.
How did you get to the conclusion that the island of the Philippines can't support (from now, and for a VERY long time in the future, even if perhaps not indefinitely) 3 children per woman ?