How many should be in a planning session? We’ve had as few as two and as many as 80.
Generally, in a non-governmental organization, we suggest the full board of directors (if there is one) plus the CEO. The board sets corporate direction in it’s planning process. That would normally be followed by a staff planning session involving the operating officials (the CEO and his/her direct reports) to operationally implement the corporate plan.
A similar pattern is used for city and county governments. A city, for example, would have a session involving the City Council and the City Manager. That would be followed by an operational planning session with the City Manager and his/her direct reports.
There are also ways to involve the workforce (maybe more about that later).