As long as you keep your child "in check" everything will be fine. I would be perfectly happy with one child, but I'm not keeping two out of the question. Whether or not it's a stereotype, whether or not it's fair, mean, ignorant, whatever, the fact is, a portion of "only childs" are brats. There are tons of brats that are siblings as well, but it's a different type of "brat."
The only-child "brat" is the one that goes monkey shit when they have to share something. The type of person who has no respect for others and simply doesn't notice when they are being inconsiderate of others in the same room. You gotta nip that right in the bud. Don't put your only kid on a pedestal.
The best example: "but it's MINE!!!!!!" when referring to having to share something. Jesus, chill out, share. It's not that big of a deal. I realize a lot of people with brothers and sisters can have problems similar to this, but it seems to be more rampant with only-childs. Coming from a larger family with 4 kids, I have a twin brother, and 2 sisters. You better believe I realize I'm not special and sharing is what you are supposed to do. Yes, it's fine if you have a slice of my pie.
Parents with only one kid: please let your kids know that they aren't special. Teach them to share, teach them to respect others, teach them they aren't the center of the universe
same goes for kids who aren't only-childs as well.