# of MySQL connections
In MySQL, there's a "max_connections" variable: https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_max_connections

In Apache (assuming you're using Apache) there's a "MaxClients" directive: http://web.archive.org/web/20160415001028/http://www.genericarticles.com/mediawiki/index.php?title=How_to_optimize_apache_web_server_for_maximum_concurrent_connections_or_increase_max_clients_in_apache

I'm not sure of your situation, but you may need to modify both of these to achieve what you need. These settings are not controlled through CraftCMS, but instead need to be controlled by editing your server environment configs.