From a private point of view, only one:
In comparing identical twins, Kohler found that mothers with one child are about 20 percent happier than their childless counterparts; and while fathers’ happiness gains are smaller, men enjoy an almost 75 percent larger happiness boost from a firstborn son than from a firstborn daughter [TC: remember the result that fathers with sons are less likely to leave?]. The first child’s sex doesn’t matter to mothers, perhaps because women are better than men at enjoying the company of both girls and boys, Kohler speculates.
Interestingly, second and third children don’t add to parents’ happiness at all. In fact, these additional children seem to make mothers less happy than mothers with only one child–though still happier than women with no children.
"If you want to maximize your subjective well-being, you should stop at one child," concludes Kohler, adding that people probably have additional children either for the benefit of the firstborn or because they reason that if the first child made them happy, the second one will, too.
I am hardly an expert in this area, but I find the logic appealing. One kid is quite able to fill your time and thoughts. I call this the "parent as empty vessel" model. The argument for more than one kid, in this view, would rest on risk-aversion and the chance that one kid might die or not work out so well.
Note the contrast between Kohler with Bryan Caplan’s theory that you should have more kids now than you want, so you may enjoy them when you are old. At that point in time, no single kid "fills the empty vessel" and so more of them are needed.
I believe that men enjoy children more than women do, as they are less stressed by worry. Whether men want children more is a different question [this last sentence has been altered from a previous version.]
The pointer is from the still totally awesome www.politicaltheory.info.