Just make a pull request on github or create an issue ticket there.
You can see some examples here.
This number is used to give the downloadable map files the correct file name. If you want for example to replace map number 7 in your world you just have to input "7". When you use the split-image feature, all map files in the downloadable zip archive will get their number according to your input: If you input "7" and split the image into 4 parts the files will have the numbers 7, 8, 9 and 10.