When a couple discusses the amount of children they would like to have (leaving aside how may G-d gives) is this question merely hypothetical? If I understand correctly birth control may not be used unless this poses a serious threat to health or life, and if this is the case one has no way of controlling when and how many children one has, assuming normal relations, is that correct?
Most authorities permit birth control under certain circumstances, even where there is no “serious threat to health or life.”
In particular, once the couple has already fulfilled the mitzvah of p’ru ur’vu, many poskim are prepared to be lenient for lesser reasons.
If a couple feels that their shalom bayis (for instance) will be adversely affected by many consecutive children, if a mother tends to have children once a year and feels that this will stretch her beyond her limits, or even for reasons of working/studying — there are poskim who will permit a couple (after p’ru ur’vu is fulfilled) to take contraceptives.
At the same time, some poskim are more stringent, and if the only reason is for “general comfort” (this is of course a broad statement), poskim will generally not permit birth control (see Rambam, Ishus 15:1 concerning the obligation of having children).
Therefore, there is room for talk of “family planning” within a halachic framework, though of course the considerations of an Orthodox Jewish couple will differ from those of others.
Please see “Posponing Childbirth”, and the interesting dicussion in the “comments,” for more on this subject.