I don't think you have to be rich- It is character building for children to learn about neccessities and wants, nobody 'needs' the latest everything, labelled clothes, lavish holidays etc, it is nice for children to learn about saving for things we want.
However, having said that I do agree with Ang- you have to be able to afford the basics, if you can't make ends meet to pay your rent, buy food and pay the bills, then you probably can't afford to have another child- because all the love in the world is not going to pay for your electricity or buy food for your children.
Sadly I have worked with some of the kids that Angelaartsstudent is refferring too, whose parents have more children for the baby bonus- who get paid their centrelink, go and buy their alcohol and cigarettes, buy the family take away and then the kids go without food for the rest of the week- those really are the people who shouldn't be having more kids.

So like everyone says, it's about balance, if you are covering you basics and you are prepared to sacrafice material possessions- then sure why not have a large family, it is an each to their own kinda situation. For me, I don't want to be rich, but I am happy with 3 kids and knowing that we have enough money to do the things we want to do.