#!/bin/bash

letters=( {A..Z} )
pwd # huh?
n=0
for file in *flac; do
   let "numz = n / 26" # you can also do this with (( numz = n / 26 ))
   let "apos = n % 26" # ^^
   unset FileName
   i=0 # use for (( i=0; i < numz; i++ )); do
   while [ $i -lt $numz ]; do
      FileName="${FileName}Z"
      i=$(( $i + 1 ))
   done
   FileName="${FileName}${letters[$apos]}"
   n=$(( $n + 1 )) # This is fine. You can also do (( n++ ))
   echo "Moving $file -> $FileName" # Or you can do mv -v if your mv supports it (-v is not POSIX)
   mv  "$file" "$FileName"
done