The first digit would have to be 1, the remaining digits, zero.
100,000,000,000
1023456789
10234567
10234567
0123456789
Since there are only five different digits, a 6-digit number can only be generated if a digit can be repeated. If digits can be repeated, the smallest 6-digit number is 111111.
102
103
The smallest 4-digit number without any repeated digits is 1023, as it starts with the smallest possible digit (1) followed by the next smallest digits (0, 2, 3). The largest 4-digit number without any repeated digits is 9876, which uses the largest digits available in descending order.
999,999,999,999
The smallest number with no digits repeated would either be zero or one, depending on the set of numbers you want to choose from.
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.