codes composed of six uppercase English characters, at least two characters are different between any two codes, how many codes can be generated?

Question

Generate a set of codes composed of six uppercase English characters, requiring at least two characters of same position are different between any two codes, how many codes can be generated at most?

such as

['AAAAAA','AAAABB','AAAACC'...]

I have tried exhausive the results, but the code is too slow， or how to optimize the code

arrs = []
totalLen = 0
def isValid(t):
  for arr in arrs:
    equalCount = 0
    for i in range(0, len(arr)):
      if arr[i] == t[i]:
        if equalCount == len(arr) - 2:
          return False
        else:
          equalCount += 1
  return True
hex = 'Z'
for a in range(ord('A'), ord(hex) + 1):
  for b in range(ord('A'), ord(hex) + 1):
    for c in range(ord('A'), ord(hex) + 1):
      for d in range(ord('A'), ord(hex) + 1):
        for e in range(ord('A'), ord(hex) + 1):
          for f in range(ord('A'), ord(hex) + 1):
            t = [a, b, c, d, e, f]
            if isValid(t):
              arrs.append(t)
              totalLen += 1
              print('    ' + ''.join(chr(ch) for ch in t))
        print('  ' + chr(a) + chr(b) + chr(c) + chr(d) + ' :' + str(len(arrs)))
      print(chr(a) + chr(b) + chr(c) + ' :' + str(len(arrs)))

print(len(arrs))
```