I am an only child, are there any problems associated with being raised that way?
no not really.  some children have half-siblings or step-siblings living at the same house or a different house may have a similar family environment to only children.
Are there a lot of only children IN the United States?  I feel like other countries usually hyave lots of kids?
Only child
An only child is a person with no siblings, either biological or adopted. 
Children who have half-siblings or step-siblings, either living at the same house or a different house - especially those who were born considerably later - may have a similar family environment to only children, as may children who have much younger siblings from both of the same parents (generally ten or more years).
Overview.
Throughout history, only children were relatively uncommon. From around the middle of the
for likely influence on the subject's Style of Life. In sum, the subjective experiences of sibling positionality and inter-relations are psychodynamically important for Adlerian therapists and personality theorists, not the cookbook predictions that may or may not have been objectively true in Adler's time.
For Adler, birth order answered the question, "Why do children, who are raised in the same family, grow up with very different personalities?" While a strict geneticist, believing siblings are raised in a shared environment, may claim any