I have to create a local network (without internet connection) with several Pis (around 20) in a straight queue. Each Pi has to connect as many Pis as possible, but the regular case is that every Pi would have one Pi at the left side and another at the right. The Pis connection between them has to be via WiFi. The WiFi has to be visible as only one WiFi among all the Pis. Every Pi has a camera and has to send an alert (when an event happend) to every Android device connected to the same WiFi as Pis are. The Android devices have an special app designed for this.
For example: If Pi number 1 creates an alert, the message has to reach to every device connected in the whole WiFi, including mobile devices connected to the Pi number 20.
My questions are:
1) Is this possible to do?
2) How can I do that?
3) Every Pi has to be a router and then connect all of Pis between them?
4) Do the Pis need two WiFis? (One for the connection between them and the other to connect de devices)