987
9876543
100000
9876543
987654321
9,876,543
I believe it is: 987,654
Multiples of 12 are even. Start with 999998. It's not divisible by 12. Try 999996. It is divisible by 12. Stop there.
The greatest possible number is 888... (repeating).
1,023,456,789
9876543
98765432