Each Coven is usually autonomous and makes their own decisions depending on the traditions it follows. You may have as many or few as you like in a Coven. Two is deemed as a partnership, and some still see thirteen as ideal. however, the larger the Coven, the greater the responsibility.
Not all Covens will have the following roles or systems of degrees as it can vary upon tradition but most commonly found are:
High Priestess & High Priest ~ Wiccan in belief, Group leaders, could have one, both or none.
The Elders ~ Usually Witches with much experience or advanced knowledge in the craft.
Summoner/Maiden ~ Understudy to the High Priest/Ess.
Third Degree ~ Wiccan Path, Minimum level needed to become a High Priest/Ess. Has studied in depth.
Second Degree ~ Witch with intermediate knowledge and would have been involved for at least two years.
First Degree ~ Witch that has been initiated but not advanced enough level of study or time passed.
Neophyte/Aspirant ~ Interested in learning the Craft or Religious Pathway or in first year of study.
Other roles could include specialists in a particular field, for example, Medium/Herbalist/Divination as well as the usual group roles such as secretary/treasurer etc. Some Covens require the Leaders or Elders to initiate new members before they can be considered part of the group or tradition, this would involve a ceremony as well as an Oath of Loyalty. The Coven will usually follow the path as taught by their Elders, although I believe this will evolve as more Covens seem to be steering towards the structured education rather than the traditional Year and a Day Wiccan initiation process.
When a Coven has grown too large to be manageable it can split, also known as Hive off. An example would be when a member reaches the highest level or degree, with leaders blessing would leave to start their own coven, or daughter coven as they are sometimes called. so, one or more witches would leave the mother coven and form their own group.