Radiopacket [20] as variable

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Arduino pro mini
Rfm69hcw adafruit

1.  What does the 20 mean in radiopacket [20]  ?

2.  How to use radiopacket [20] as a variable?
2a.  char radiopacket[20] = "Hello World #";
     When substituting a variable for "hello world" it says to put quotes so its a string.