I'm with Rebis. Everyone is an ambivert; what makes us more introverted or extroverted depends on our tendencies of how much social interaction we want.
The average human will need time with others, and need time alone. People who live very social day-today lives may think they're an introvert because they find themselves needed solitude at the end of each day. Others who work very independently, or don't happen to interact with others a lot may consider themselves extroverted because they more readily feel the drain of being alone so much.
I think whether or not we exhibit introversion vs. extroversion is a mix between our biology + environment. If you're someone who's really on the line, you'll identify more with introversion or extroversion depending on your circumstances. In any case, it's healthy to be balanced.
For determining whether or not you're an INTP or ENTP:
There are more differences between these types than one being the extroverted version of the other. Since you know that you tend to be pretty balances with your E and I, start comparing other traits to see what strikes a chord in you the most.
For a long time, I was unsure whether or not I was an INTP or an INTJ. What made me sure I was an INTP was reading about the progression of thought. I could be really bullheaded about some things and act very judgmental, but I had the same troubleshooting habits of INTPs. I constantly checked and reevaluated whether or not I had any biases in making a decision, whereas my INTJ friends took more time to make a decision, but rarely ever re-accessed themselves afterwards.
