642
All numbers are made with digits and there is no limit on how big they can be. So there is no greatest number.
0123456789
999,999,999,999
§ In comparing two whole numbers, the one with the most digits is always the greater number. § In decimals the number with the greatest number of digits is not always the greatest.
The greatest 4-digit number with no repeated digits is... 9876
9,876,543
98,764
987654
98,764
9876543
98,764
98765
987,654
987653
94205 if digits are not to be repeated and 99999 if digits can be repeated
The largest ten digit number with no repeated digits is '9876543210'.