This is a great question! The steem ambassadors should simply be the volunteers who the community decides have done the best work for promoting steem. In my mind this means that there is no term, as long as the ambassador continuously demonstrates great quality promo work or ambassadorship to steem. In theory there is no limit to how many ambassadors we can take on. We will start initiatives to un seat ambassadors should they been caught abusing the system or for some reason stop supporting promo-steem. The question for unseating will still be put to the community to decide however. We do not want central planning.