138,146
We live in base 10 with 10 different characters as numbers. When we reach 9 the next number is made out of two characters, 1 and 0. Base 1000 would mean there's 1000 characters, and when we reach our 999 the next one (one thousand) would be written as 10. So assuming there's a specific character for a number 138, let's say it's a, then 138,138 would be written as aa.
I'm sorry I'm confusing most likely.