I come from a family of 4, and I think we're all pretty close, except child #3 (my sister) has always had issues with child #4 (me.) We grew up playing together constantly, despite a 4 year age gap, but her personality is a manipulative one, and she's manipulated me since she learned how. She extorted my allowance from me, and stole my toys, and all sorts of other things. In high school, she borrowed my clothes all the time, but refused to let me borrow hers. She got up earlier, so she could sneak in my closet without my knowing it.
Later, when I was 11-ish, I got to know my oldest sister, 10 years my senior, when she was home from college for foot surgery. That's when I realized I hadn't ever really known her before that.
We're all very different people. I think my brother and I are more alike than any of the rest. My oldest sister is all about fashion and Home Beautiful magazines, my brother is a respected medical researcher and professor, my second sister is an artist at heart who takes care of wayward children by running a group home. I'm a biologist/artist type. But we all love each other very much, and would do anything for each other.
My husband's family is way different. They're really a lot more similar to each other than my siblings are, but the strife in their family is terrible. They picked on their one and only sister so much, doing things to her that I would never imagine doing! Like tying her to a chair and leaving her in the alley for trash pick-up. She was scared to death! They tied her dolls to the ceiling fan blades and turned on the fan to torture her. She got out of the house the moment she graduated from high school and doesn't come back when her brothers are there, if she can help it. I was shocked when I heard their dynamics. They call each other fat, stupid, ugly, and worse. I bet if my husband keeled over this second, they'd all call to say they're sorry, but none would shed a tear or come for a funeral. Nor would he go to one of theirs.
The point of my long long answer is that I don't think sibling closeness has so much to do with how many children there are as how they are raised.
But I will tell you that two kids is infinitely easier then 3! Our 3rd is 10 months old now, and life is hard sometimes. Of course, we love him dearly and couldn't imagine life without him, but it's a lot harder to get them all out to the car, get them all dressed, get them all fed, bathed, to bed, etc. But that, I hear, is just a phase. Just one more stage of life that we'll miss when they're older and more independent. But right now, I just have a headache!
That said, we're planning on four. It may be hard, but it's worth it.