You will probably find that if you do 'which httpd', you won't get anything in return, either. That's because httpd in not in your path.
Do 'locate httpd | less', and scroll through the output. You will probably find the httpd executable in /usr/sbin.
Now, re-issue the command with full path to httpd, like so:
/usr/sbin/httpd -v, and you will get the output you seek.