Define "word". However, strings separated by space:

Print number of words:

    wc -w <filename>

Print words with counts in decreasing order:

    cat <filename>  \
    | gawk '{for (i=1;i<=NF;i++) print $i}' \
    | sort          \
    | uniq -c       \
    | sort -rn      \
    | less

There are optimizations to be made, but that would be my go to solution in the first instance.