Configure your Pool pm.*
directives & reload/restart PHP - http://www.php.net/manual/en/install.fpm.configuration.php
pm.max_children int
The number of child processes to be created when pm is set to static and the maximum number of child processes to be created when pm is set to dynamic. This option is mandatory.
This option sets the limit on the number of simultaneous requests that will be served. Equivalent to the ApacheMaxClients directive with mpm_prefork and to the PHP_FCGI_CHILDREN environment variable in the original PHP FastCGI.