This is what the Catechism of the Catholic Church (2368) has to say on the issue of “the regulation of procreation”:
A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. Moreover, they should conform their behavior to the objective criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life, the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts criteria that respect the total meaning of mutual self-giving and human procreation in the context of true love; this is possible only if the virtue of married chastity is practiced with sincerity of heart.
This is not as specific as one might like when trying to determine whether one is using Natural Family Planning for just reasons, but that is because the Church cannot possibly envision all of the particular circumstances in which couples might have just reasons for delaying or seeking to end through just means (i.e., NFP) the birth of children. So long as one remains open to the possibility of life through NFP, there may be many just reasons why one may not actively seek to have more children.
If you and your husband need help in this discernment process, you might consult with an orthodox and balanced priest or spiritual director, or you might call Gregory Popcak’s Pastoral Solutions Institute for help or for a referral to someone in your area. I also recommend the following article:
God, Sex, & Babies: What the Church Really Teaches about Responsible Parenthood by Christopher West