A lot of people believe 2 kids maximum because then you are only replacing yourself and your partner, so you arent contributing to the over population of our planet...just something I read recently.
I think its a personal choice upto each couple to decide how many they can provide for financially, have enough time and energy for, afford a big enough home for and all those types of things. Otherwise I dont think its right to plan a baby. There are pros and cons for both small and large families, do what you feel is right for your personal situation I guess.
Morgan