Since /status page don't show pm.max_children parameter, at current realisation impossible to get if limit is reached. This is most important thing for me. Maybe it will be possible with agent plugin, nvm.

I assume add macro to template :

{$PHP_FPM.PROCESS.MAX} - current pm.max_children

{$PHP_FPM.PROCESS.MAX.TRESHOLD} - % of processes active utilization

trigger:

PHP-FPM: {$PHP_FPM.PROCESS.MAX.TRESHOLD}% Max processes reached

100*last(/PHP-FPM by Zabbix agent active/php-fpm.processes_active)/{$PHP_FPM.PROCESS.MAX}>={$PHP_FPM.PROCESS.MAX.TRESHOLD}