I have two. I was really very unsure about having a second. My first was such hard work. I didn't think I could do it again. My marriage had taken a massive hit, the adjustment to being parents having been in a couple for years was just massive.
In my heart I knew I wanted to try for another baby. We'd had IVF for our son and we had frozen embryos to use. That helped me make the decision as I didn't want to 'waste' them if you see what I mean. In my head I was totally unsure that it was the right decision, all the way through really until she was born and absolutely stole my heart. The utter joy she brings to us, and to her brother especially, is off the scale. I couldn't have pictured it before she arrived. No matter how shitty your day, watching them giggling away together makes everything feel better.
Yes it's hard, my favourite saying is 'if it's not one, it's the other.' They like to tag-team with whining, falling over, needing nappies changed, waking up through the night... it's a sibling skillset I hadn't anticipated 
It is easier in so many ways though - they entertain one another and keep each other company. DD has hardly had anything bought for her as she plays with his old toys, in his old cot, car seat, buggy etc. You're already in the routine of up early, milk, nappies, activities, naps, early bedtimes, clean up, sit down exhausted, do it all again tomorrow. 
I was very, very unsure of having a second. I just didn't think I was capable of it. Now... I'm sort of considering... trying for a third
I completely understand why people stick with one child. It's lovely in so many ways and makes perfect sense. Don't ever feel bad if you decide just to have one.
PS I'm one of 6 kids, very close to my parents and siblings and never felt I wanted for anything (time or material) growing up. It can be done!