One. Anything more increases the total population. We need it to decrease and if it doesn't because of intelligent behavior then natural law will decrease it through disease, war, pollution, hunger and other mechanisms. That may take a few decades but in many places it has already begun.
This is illogical. If 'intelligent' people realised that having fewer children would be beneficial for the world, the result would be a smaller percentage of intelligent people in the population, which would in turn lead to a larger increase in population. The most intelligent people will want to have more children, not fewer, because they are more likely to benefit the population as a whole, for whatever reason, especially if they are Christians.