The number and total size of the files is irrelevant.

All new files will not be affected by whatever happens inside the for loop.

If you want them to be affected as well you could put the for loop e.g. inside a while loop.

Suppose your for loop works on files with the extention ".txt".

while [ `ls *.txt > /dev/null 2>&1; echo $?` -eq 0 ]
do
for FILE in `ls *.txt`
do
do_whatever_needs_to_be_done
done
done

The while loop makes sure that as long as *.txt files are present in the directory the for loop will be executed over and over.

Ofcourse this process will come to an end at sometime as well, when no more *.txt are present. New *.txt files arriving shortly after are not affected either.