Group of witches.
The average coven consists of about 4-5 people, though 13 is the traditional limit to size and 3 the minimum.
Joining a Coven
There are more solitaires, at least in the US, than coven members. Joining a coven is like joining any other group - you look for common goals, common values, and beneficial energy between the members. The smaller the group, the more important compatibility becomes, and covens are small groups.
Many people think, after reading older books, that joining a coven is a natural outcome of being Wiccan, and that covens must be sought out. This is not true. Some people do benefit from close-knit, regular interaction, and covens work well for those kind of people. However, there are many who are content working on their own, and many others who only want community interaction on a when-you-feel-like-it basis. Being in a coven means some degree of dedication to the well-being of that group, and of its members. The level of dedication requires is going to depend on the group.
The traditional saying is, when the student is ready, the master will appear. This saying goes for much more than spiritual teaching - the lesson will appear when the person is ready for it. If a coven is sought out and not found, don't worry about it. It'll happen when it happens. And the lesson learned might be that you aren't really looking for a coven after all.