In the dragonshout piece at game informer they said there are 20 shouts. My question is do we have to get all 20 to complete the main quest? Hopefully not. And can we get any by not doing any of the main quest or are they only tied into the MQ?

I would imagine that the first shout and, thusly, the ability to obtain any subsequent shouts would be tied into the MQ. There may be a few shouts tied into the MQ, but surely the rest will be optional. You know, obtained from defeated dragons and cave walls and all that.

You think we'll have to learn all 20 to complete the MQ?

I doubt it would be "You must go and learn 20 shouts before you can slay Alduin.

Though I imagine it would be quite advantageous if you DID have them all.

I would like to think you could defeat most dragons without using shouts however (but it would be quite harder) and that at least a quarter of the shouts were NOT obtained through the MQ, but exploration, and random dragon encounters.