How many apostles are there? Are there 12 apostles? Are there 70 apostles?

When I was growing up and learning about Jesus, I found that he had 12 apostles. I was looking at a page about Mark the Evangelist recently and it says that he was one of 70 disciples. On that same page about the about 70 disciples there is a picture titled "Icon of the Seventy Apostles.".

Are there 12 or 70? Are the 12 somehow separate from the 70 or are they part of the same group?

Is there anything in the Bible I can read that deals with this?
It probably depends on what the specific church calls an apostle, but at least we know that the "seventy disciples" are a distinct group of the twelve (apostles).

From Luke 10:


  And after these things, the Lord did appoint also other seventy, and
  sent them by twos before his face, to every city and place whither he
  himself was about to come, then said he unto them, `The harvest indeed
  [is] abundant, but the workmen few; beseech ye then the Lord of the
  harvest, that He may put forth workmen to His harvest.


The twelve were mostly with him, as far as I remember he was alone with them at the last supper. This shows they were, somehow, special - distinct from the seventy.

LDS view:

In the Church of Jesus Christ of Latter-Day Saints, we have a quorum of twelve apostles and quorums of seventies that assist them. I would (We) see the purpose of the original seventy as similar: Assisting the twelve, going out and preaching. Because really, twelve people is not many.