9876543.
642
There is only one number and that is 123456.
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
The greatest 4-digit number with no repeated digits is... 9876
9,876,543
98,764
98,764
987654
987,654
987653
9876543
98765
98,764
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'.