Well I'm 21 and he's 24 so if we needed 10 years to be ready again we could still most likely have another. I've just heard so many stories of people just knowing they wanted to have another (maybe not right away but eventually) as soon as their LO was born. My parents even joked about us purposely having 'Irish Twins'.
And many of them do that and then wonder what the heck they were thinking. Many of them also realize after a few months that it was a completely insane idea (for them) and are glad they didn't do it. One of my sisters was so positive they were going to try for another one ASAP. They decided that while she was pregnant and were saying it the first few weeks after the baby was born. By about 4 months later they had started saying she was going to be an only child. lol People change their minds a lot. Don't worry about it. Having just one is FINE, and having more when your one is several years old is also fine. :)