I got married in my early 20s, have been married 15 years, and have three children (there was also a recent miscarriage). Our first child is special needs and we have a substantial gap between our second and third children (7 years). I think my Catholic peers tend to have somewhere between 4-7 children.
I think different people are going to have different results with NFP, due to differences in fertility, health, patience, level of motivation, age, level of science literacy, etc. Based on what I read here, I think the trickiest times are probably breastfeeding and perimenopause, as one is working with much less information. Ironically, the more fertile one is, the easier it is to do NFP.
I would certainly caution any newbie to avoid the following approach: “put things in God’s hands” and then freaking out when health problems or other serious reasons arise and it looks like the couple is facing either prolonged total abstinence or danger to life and there’s been no track record of successful spacing with NFP. I strongly advise making some allowance for the unforeseen. For instance, if you have already had two c-sections and it looks like further deliveries will also be c-sections, I would argue that that is a serious reason to space out further pregnancies. I know that some women manage to have a lot of c-sections, but it seems wisest to me to not presume on being able to safely have half a dozen. Likewise, there are a number of disabilities that will not be immediately obvious to new parents and it is actually not uncommon to wind up with multiple special needs children in a good-sized family. I would not encourage anybody to bet on every one of their children being at least physically and mentally average.
There’s a parable in the gospels where Jesus says, “For which of you, desiring to build a tower, does not first sit down and count the cost, whether he has enough to complete it? Otherwise, when he has laid a foundation, and is not able to finish, all who see it begin to mock him, saying, ‘This man began to build, and was not able to finish.’” There are occasionally sad cases, for instance the Torodes, the Protestant and then Orthodox couple that wrote The Open Embrace and eventually divorced, having had four kids.