I believe it is the lalafells ability to use tools and think critically and abstractly that is what leads them to being a populous race.
I think it's our ability to make lots of sexy time that led to us being a populous race. ALL RIGHT!
Half the Fates in Ul'Dah are about having to deal with some giant monster or other that keeps eating Lalafels, so I think the answer to your question is "A lot".
Every time you lose a FATE, Lalafells get busy replacing the ones the monsters eat.