title: numChildren in Firebase rules
body: <p>I have <code>/members/($chatId)/($userId)</code>
   firebase datas.
  (chat room with some users)</p>
  
  <p>How to prevent write new userid, if the count of users in chat more than any number (ex. 6)? Of course, how to do it using firebase security rules. If you write solution in firebase bolt is more awesome ;)
  Thank you!</p>