The President of the Quorum of the Twelve oversees all of the Stake Conferences. He assigns the general and area seventy visitors as well. When there are no presiding visitors, then the President of the Twelve designates the Stake President as the presiding authority.
Visits from a member of the First Presidency or Quorum of the Twelve are increasingly rare at individual stake conferences and generally only happen when it is time to create a new stake or ordain a new stake president. Even in those cases a General Authority Seventy or Area Seventy may officiate instead of an Apostle.
On occasion there might be more than two stake conferences in a year. That may happen when a Stake President needs to be replaced.
Districts in the Church are found where there are too few members to comprise a stake. District conferences follow the same format as stake conferences with the following exception: the Mission President presides over district conferences; the district president does not preside. Sometimes a district participates in a regional or broadcast conference.