Assume my website (developed using Craft, of course) is expected to have 1000 concurrent access. How many MySQL db connections would be required to open? Does Craft control this or does it depend on program code?
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.