Most of my friends are extroverts.
I am  definitely an introvert, not outgoing at all. 
I think I am probably in between.
So you don't like being alone but are good in crowds? Or how do you characterize your introversion/extroversion?
It depends on the situation. I am more comfortable in smaller groups of close friends.
I guess a lot of people feel they have a mix of both personality types.
That makes sense.
Also most people think that there has to be one side that is more dominant than the other.
I could see that being the case. It does seem like a person would lean more one way than another.
There is one idea that pins introverts as being more emotionally and inner driven while extroverts are more concerned with the outside world and things they interact with.