Whilst there isn’t a lot of research specifically on the correlation between number of people and productivity in meetings, many seem to agree with the George Miller Magic Number 7 +/-2 principle. 

George would say that when it comes to a meeting, it is starting to get unwieldy if you have more than 7 participants.  He would suggest 5-7 people are best and if you MUST up to 9.  Beyond that it becomes unwieldy..

Jeff Bezos, CEO of Amazon says that the more people there are in a meeting, the less productive it is.  He has encouraged people in his organisation to use the ‘2 pizza rule’.  If 2 pizzas wouldn’t feed the group, then you have too many people.