987654
It is 987654321.
98
A palindrome reads the same forward and in reverse. This tells me that at leastthe first digit and the last digit must be the same. So it's not possible to have a6-digit palindrome "with no same digits".The largest 6-digit palindrome, with just enough repetition of digits to make it apalindrome and no more, would be 987,789 .
9876
2
999,987,654
99,998,765
99,998,765
9998888
Well, honey, the largest 12-digit number with repeated digits allowed is 9,999,999,999,999. I mean, unless you want to get fancy and start throwing in letters or emojis, but I doubt that's what you're looking for. So there you have it, the biggest 12-digit number you can get with the same digit repeated over and over again.
Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.
There are 900 of them.