How many people is a table?
Six is a conversation. Twelve is an event. Thirty is a wedding you have to cater. Where the line sits, and why it matters more than you think.
Every group we have watched has a size it works at, and it is smaller than the size it aspires to. Groups grow because inviting someone is easy and uninviting them is not, and then one evening the thing everybody liked about it has quietly gone.
Under eight
One conversation. Everybody hears everybody. Cooking is easy and the cost per head is high, because you are not buying anything in bulk. This is the size most groups are happiest at and the size they leave first.
Eight to fourteen
The table splits into two or three conversations, which is fine, and the cooking gets more efficient, which is better. This is the range a home kitchen handles without anyone suffering. It is also the range where the admin starts to bite, because fourteen people cannot be coordinated in a thread.
If a group is going to need help running itself, it usually discovers that somewhere around eleven.
Over about sixteen
You are catering. That is not a criticism, but it is a different activity with a different skill set, and one person doing it for free every month will stop. Groups this size survive when the cooking rotates properly, when the cost is worked out in advance, and when somebody is willing to say no to a fifteenth guest.
The useful rule
Set the number of seats before you set the date, and hold it. A cap is not unfriendly. It is the thing that lets you say yes without hesitating, because the answer to the person you cannot fit is not "no", it is "next month".