According to Darwin, what size litter would be most beneficial for a rat in order to ensure survival and why?

Based on Darwin, the size litter of rats must be large for them to ensure survival. Having large size would facilitate co-evolution, random mutation and natural selection that will allow adaptation and survival of rats on the new environment. Not to forget that rats are highly intelligent and have good senses of hearing, sight, and smell, and can eat a wide variety of food.