CYGWIN( UNIX):How can I use variables count how many times grep finds a "yes" or "no" in the files in my directory?

IN CYGWIN( UNIX) I am searching for the words "yes" "no" in files my directory. each file will only say "yes" or "no". How can i use variables YES=0 and NO=0 to count how many times grep finds a "yes" or "no" in the files in my directory?