4, since by definition a byte is made up of 8 bits.
Eight bits are 1 byte. The word 'byte' was coined from the word "by eight" - 1 bit multiplied by 8.
Use data-type 'long long' or 'int64_t' (from inttypes.h)
Assuming a byte is 8 bits, then a 2 byte word is 16 bits. Therefore there are maximum of four 2 byte words in a 64-bit variable. Note that a byte is defined as being the smallest unit of addressable storage. As such there is no official standard that dictates its length; it is entirely hardware dependent. Some systems can address at the bit level, thus a byte would literally be just 1 bit in length. Although most systems today use an 8-bit byte, this is not always the case thus the term octet was defined to specifically mean an 8-bit byte.
A double byte is two bytes.
As a 21 byte array of type char (including 1 byte for the null terminator).