Most people are doing something morally wrong by not having children, or only having 1 child. That seems like a reasonable conclusion to me.

However, I wouldn't push the point. My view is that "life is good so having children is good" but not "you must have as many children as possible." There are various ways I might justify this philosophically, but I would just say that parents have a special obligation to their children and, if you have too many children, it's impossible to fulfill that obligation. So you should have as many children as you can effectively parent.