Php-fpm/fcgid based on client php thread (using nginx/lighttpd/nginx or httpd with php-fpm/fcgid php-type). It's mean every client will be php threads/childrens allocation. By defaults KLoxo-MR allocate 6 php threads/childrens. So you can calculate how much RAM needed for 50 clients.
If using httpd with ruid2/suphp, no php threads allocate for every client. Php threads using globally. It's that mean php threads as share for all clients.