How many children should I have?
Not too many! God has not commanded that you have any. Elijah, Anna, John, Jesus, and Paul did not have any. Society is drastically changing and making it much more difficult to train children, and wise men will adjust accordingly, just as Paul taught the Corinthians about marriage itself (I Cor 7:26). Children absolutely are a burden, no matter what some sentimental and ignorant person may say, just like marriage is a burden (I Cor 7:29,32-35).
Much more about birth control and family planning: Birth Control.