Hmmm.... just a guess, but...
The file names are consecutive by number.
You have 1 (one) more file than suggested by the largest number.
Then, I suggest that you may have two files with the same sequence number (maybe one with an additional -1 in the name, automatically renamed because you have two with the same sequence number). Maybe a one-time glitch in your renaming utility
More than 18,000 files in a single folder, finding one file that's slightly different name - seems a daunting task !