Arduino - convert int to byte array[16] - RFID write data

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4 
byte number[16] = {"100"};

Declares and initialises an array of 16 bytes the first 3 of which contain the numbers 49, 48 and 48 respectively which is the ASCII representation of the digits

int var = 100;
byte number[16] = {var};

Declares and initialises an array of 16 bytes the first 3 of which contain 100, 0 and 0 respectively (if declared globally) and 100 in the first and random values if declared in a function

What is it exactly that you are trying to do ?