How many people should be in a meeting?  Well if the meeting concerns making a decision, research suggests that seven or less is usually the correct answer. Each person beyond seven who attends the meeting reduces decision effectiveness by around 10%.  Thus, meetings with 17 or more attendees usually can’t make any decisions.

But not all meetings are for decision-making.  Another rule of thumb is the so called 8-18-1800 rule:

• If you want to make a decision or solve a problem, keep the number of people in the meeting to less than eight.
• For brainstorming, go as high as 18 people.
• If you want to provide updates or communicate information, go as high as 1,800 people (or higher).

Sources: Bain & Co Research:  http://www.bain.com/publications/articles/decision-insights-9-decision-focused-meetings.aspx

HBR: https://hbr.org/2015/03/how-to-know-if-there-are-too-many-people-in-your-meeting