Six is a Perfect Number, mathematically speaking, and is a good power number. Twelve is also a good powerful number. A Dozen is an iconic grouping, and is a commonly occurring number in some branches of occultism.
In a perfect world, all other things being equal, a coven of twelve would give you a greater amount of total power when you had the entire coven acting in concert -- simply because there would be twice as many of them.
Unfortunately, this ISN'T a perfect world. Personally, I'd rather have just ONE partner whose skill and experience are comparable to my own than eleven other people who can't even raise a proper shield on their own.
In addition to the quality of the members, you also need to factor in everyone's availabilities. The difficulty of arranging a full meeting of any group of people increases exponentially as you add members. For that reason alone I would probably choose a team of six over a team of twelve if being able to meet in person regularly were important to me.
And of course, as has been noted, there's the social aspect of things. Getting most magic users to work together seems to be like trying to herd cats into a bathtub, unless they already all think alike anyway. If you can find six like-minded people, but not twelve... well, there's your answer.
If you want just MY opinion, I'd say that if I'm looking for a study group I'll take as many people as catch my interest, and if I'm looking for a group to try and coordinate schedules with it will be the smallest group that I think can accomplish its goals.