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 children were relativily uncommon throughout history 
Why is that?  I would think having less mouths to feed would be a good thing?
well starting at around the 20th century, birth rates and average family sizes started to fall sharply due to the costs of raising children and women having their first child later in life.
Yeah I guess that makes sense.  Women are more involved now in the workplace, so they'll want kids later as well.
20th century, birth rates and average family sizes fell sharply, for a number of reasons including increasing costs of raising children and more women having their first child later in life. The proportion of families in the U.S. with only children increased during the Great Depression but fell during the Post–World War II baby boom. After the Korean War ended in 1953, the South Korean government suggested citizens each have one or two children to boost economic prosperity, which resulted in significantly lowered birth rates and a larger number of only children
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