Why can’t more than four people have a conversation at once?

There’s evidence that this four-person limit on conversations has been in place for about as long as humans have been having chatting with one another. Shakespeare rarely allowed more than four speaking characters in any scene; ensemble films rarely have more than four actors interacting at once. But why do we max out at four?

via https://qz.com/work/1388086/why-cant-more-than-four-people-have-a-conversation-at-once/