Many people have a number in their head for how many people to include in a focus group.  One of the great things about qualitative research is that there is no one – or ‘right’ – answer to this question.  If you ask five focus group moderators, you may get as many answers. 
Anything larger than two consumers in a room with a moderator would constitute a focus group of some sort.  Three people in a focus group make up a triad; four make up a quad – which is sometimes called a mini-group; five consumers in a focus group is generally a mini-group; and anything larger than five people would constitute a good old-fashioned focus group.
Eight people in a focus group room with a moderator is often a magic number, but I occasionally have clients request as many as 12 people to be recruited for one focus group.  There may be some fitting analogy out there to recruiting a jury of one’s peers to come together to discuss ideas for branding, positioning and strategy of a brand.  However, this large number exceeds anything that would constitute a productive focus group session.
The most important thing to consider when setting up a focus group for your brand, product or advertising idea is that you want people to bounce ideas off each other and grow them.  It can be very difficult for a large focus group to come together as a team and begin to work in tandem with one another.  Also, a large number of focus group participants tends to have more group dynamic issues.  Of course, the focus group moderator is there to keep those dominant respondents in check; but when there are 10 people (or more) in the room, it is much easier for someone to ‘hide’ in the background.  Ultimately, clients can end up feeling like they did not hear from everyone when the focus group has so many people in it, and that is not good for anyone. 
Another dynamic that occurs with a large focus group is that people often end up answering the moderator’s questions – but without enough follow up probing.  This is because the more people you have in a focus group, the less time you have to engage the group in discussion.  The moderator is left in a bind – either you get depth with some people or on some issues, or you hear from everyone, but just in a perfunctory fashion.  It can almost feel like the consumers are participating in a live survey, which will not help you meet your research objectives, especially when it comes to qualitative research.  There is a time and a place for surveys, but the focus group room is NOT where you want to see that happening.
So how many people are optimal for focus group participation?  Of course, it varies from study to study, but my ideal number for a traditional focus group is six participants in the room with the moderator.  Generally, we recruit eight or nine respondents to come in for the focus group, and from that set of people, we select the best six for the conversation.  Ultimately, it’s OK to go with seven or even eight respondents in a focus group, provided it is a two-hour group, but six really does seem to work best.
The reasoning for six participants in the ideal focus group:  it allows the group to form a solid and meaningful working dynamic.  They can get to know each other, and ultimately they can bounce ideas off one another and create new ideas without feeling marginalized.  Additionally, not only do the respondents get to know each other, but the clients also get to know their consumers in a more intimate way.  This level of depth that six respondents provide allows us to probe more deeply and ultimately uncover more insights for brand and advertising development.
For shorter focus groups that are more specific in nature, I have had good results with a 60-minute focus group of four respondents (recruiting 5 or even 6, if budget allows).  Having four people in this amount of time has worked well in that you get to hear from everyone in depth and also create some point-counterpoint discussion.  This focus group methodology works particularly well with advertising development, but I have also used this on an exploratory basis.
Triads – or focus groups containing three participants – are popular among clients.  When a client wants to downsize a focus group from the traditional 6-8 people, many feel that a triad fits the bill.  You can get to a deep level of intimacy with consumers, and have them debate the different sides of an issue or an idea.  Optimal recruiting for a triad style focus group is to recruit five respondents to seat three; however, you can pull it off with four recruits if your research budget it tight.  As with the four-person focus groups, or quads, you should aim for a 60-90 minute focus group.  With a group this small, you can achieve a lot quickly.  Also, as with quads, your objectives will most likely be clear and focused, so you may not need a full two hours to work through all the issues and materials during the focus group.
Ultimately, you need to look at your research objectives and figure out what plan of attack will best serve your needs when it comes to qualitative research.  Just remember, qualitative research prides itself on a ‘less is more’ philosophy, so do not feel compelled to load the room with consumers because you feel you will get more out of the focus group.  That strategy just may backfire on you.  With a fantastic focus group moderator and researcher and solid recruiting, you will get to the bottom of your issues in an efficient fashion.