I'm reading this thread with interest. I'm currently pregnant with baby number 1, but due to many reasons, not least our ages (37 and 41 respectively) we've debated whether to leave it at just one. Especially if the first one turns out amazingly. Quit while you're ahead, and all that!
The heartbreaking post above about the lady whose friend had an amazing life when her daughter was an only child, and then had a second child, with serious autism, that basically ruined her marriage, her career and her relationship with her first child, is a prime example of why we might be tempted to stop at one.
I know a few who have stopped at one, some through choice, some not. In at least one case it was I suspect partly because of a horrific birth. Nobody should judge anyone for not wanting to go through that twice.
Most only children I've known have been very grown up for their age - lots of one on one adult conversation from a young age. My husband likes that and considers that quite a compelling argument for it too.
There are no across the board right or wrong answers here. You have to do what's right for you, which in your case sounds like sticking at one.