The number of children you have is your choice. Just like the decision of whether or not to have children.
Like Lori said, there are plenty of only children that are fine, uptanding people.
By choosing to have only one child, then you don't have to "split resources". That child gets undivided attention, monetary benefits, time - you don't have to worry about getting child 1 to soccer, child 2 to cheerleading and child 3 to scouts all at the same time! (I've tried to pull this trick before, LOL!)
Yes, your child will not have a "built in playmate" - but within just a few years, he/she will be starting school and will have aounrd 20 playmates!
There are pros and cons to both sides. The ultimate decision is yours. Do not let anyone else bully or gult you into a decision that you are not comfortable with.
I am very happy with my 3 kids, I probabaly would have had 1 more, if it would not have been medically dangerous for me (needed a hysterectomy instead). One of my best friends thinks I am insane, LOL!
Sometimes I agree with her. She has one child. She can spend more one-on-one time with this child, I envy that at times, but I love all the different personalities of my VERY different children.
We are different women, so we have different wants and goals in life.
You have to decide what is best for YOU. Do not let anyone ever make you feel bad for that choice.