I still don't know what you mean by [filename], you can't pipe to a filename. You would need something like this:
ls -1 /path/to/dir | wc -l | mail -s "subject" "email@example.com"
However, that *still* won't work because you can't embed the logic to check for 10 into a cronjob. You have to make a script. Put it in /usr/local/bin. Make sure the script does not write to stdout, or redirect stdout when calling the script.