I see, that only 6 receivers can be connected with the transmitter,
That depends on how you define "connected". It is only possible for one transmitter to, at any one time, send data to up to 6 other devices (one per pipe). However, that is something that is very rarely done.
In practice no more than 2 pipes are needed for you to communicate with ...
5 bytes = more than 1 million million unique addresses, although not all addresses (as the nRF24L01 datasheet points out) are advised, for instance because they mimic the nRF's preamble packet.
So what is the purpose of addresses?
The purpose is to ensure that radio transmissions are received at the intended radio module. Consider a radio network where you ...
Remove the & in the master code line, change
message is an array, so either you should pass the address of the first character (&message), or simply pass message.
Another problem is sending a String in the slave code (using type String). It's better not to use ...