I only have two children by choice, so I can't tell you what it's like to have 3 or more, but I can tell you why I only have two. There are several factors such as hating being pregnant, age, etc., but my husband and I decided from the beginning we would only have two children because we really love to travel and we want to share that with our children. Economically, adding another child makes things a lot more difficult unless you're really wealthy. Extra plane tickets, admission tickets, another room instead of sharing a room, larger vehicles, etc. There is also the prospect of extra college tuition, extracurricular expenses while in school, preschool tuitions, etc. I'm throwing this all out there because I'm astounded at how many people never seem to consider the financial aspect of having kids! I'm not trying to say you haven't or that it's an issue for you, but it is for a lot of people -just food for thought. If you're wealthy or you don't like to travel or you don't foresee doing a lot of extras like trips to museums, theme parks, water parks, etc. then it's probably not as big of a deal, but we liked the ease of a family of four. You'll find lots of memberships to places like zoos, etc. are geared for families of 4.
Also, I'm a big believer in replacing yourself and not adding to the earth's overpopulation problem. It's just a thought, but we are killing our environment in a lot of ways, and overpopulation is one of the biggest factors.
AND -there's no guarantee on not having multiples! You can get pregnant with twins or even triplets at any time -especially if you're over 35, have a family history, have a larger frame or are of Latin or African descent.