We cannot look at your computer's files and see how the number occurs in the file, since you have not seen fit to give any example, so only very general advice will be possible. A possible approach might be
Choose a temporary file name. If the file exists, delete it.
Iterate through the file to be processed using FOR /F, checking each line using findstr to see if it is the one containing the number. Note that blank lines and lines starting with a space or a semicolon will not be processed.
If it is not the number line, write it out straight away to the temporary file using the append redirection operator.
When the line containing the number is reached, process that line using FOR with a suitable set of delims= and tokens= directives to isolate (1) the number (2) the rest of that line.
Put the number into a variable, increment it using set /a then write out the complete line with the incremented number in place of the previous one again using the append operator.
When all the lines of the original file have been processed, delete the original file and rename the temporary file with the original file's name.