There is a HUGE difference between raising one child as opposed to raising more, don't fool yourself!
I have three, plus a nephew we just took guardianship of, but my oldest was 4years old before we had more kids, so I had a little experience raising an only for a while, and due to the gender/spacing of the other kids, really kind of raised DS17 as an only in a lot of ways.
I think the biggest difference between raising one and raising more than one is setting limits. When we just had one, we could give him one more cookie....break the "no toys at the store" rule....let him make the choice on everything from what movie to what treat to what dinner he wanted, to staying up just a little longer, etc. No harm in it since there wasn't anyone else (sibling wise) to protest or have to deal with. It was so much easier to just go with the flow. HOWEVER, that was not the best thing for our stubborn, eye for an eye, black and white perspective child! He needed boundaries and to be able to predict what was going to happen Every Single Time or else he'd debate us to death (and trust me, I used the words "SHUT. UP!!" way sooner and much more oten than I ever wanted to, and the kid to this day at 17 still hasn't learned to be quiet and not argue!) Lol
The other thing I realized after I had more kids was that I tended to perceive everything DS17 did as the "benchmark" of standard and held all kids that I came across to that standard (silently!) If he potty trained at 26 months, what was wrong with that kid over there still in diapers at 3? DS walked at one...and that baby is still sitting around at 13 months. DS didn't get teeth until 9 months, so that baby over there looks weird with a mouthful of teeth at 5 months!
After I had #2 and #3 and learned firsthand how DIFFERENT each and every child truly is, I was able to adjust my "my kid is the true perfect one and everyone else's kid is behind/weird/overachieving/etc" attitude and get grounded in reality. Many moms who I've talked to who have a big age gap between their first and second say they did the same thing. So if you are only having one, just make sure you remember that whIle your child is perfectly amazing in YOUR eyes, normal is relative, and amazing has a way of blinding you sometimes. Keep it real!