I am going to develop a call manager system website that will communicate with Asterisk server(s).
It will be developed in PHP, I have been googling around and I notice there are a few opensource asterisk PHP library like
PHPAGI is certainly the "go-to" package for what you're looking to do. You'll likely want to build your own custom library on top of PHPAGI to give you higher level functionality for your particular needs.
If your PHP will reside on a different server than the Asterisk instance it is controlling, you'll want to specifically use php-fastagi. It implements the Asterisk FastAGI interface and makes it more accessible in PHP land:
FastAGI Reference: http://www.voip-info.org/wiki/view/Asterisk+FastAGI
PHP-FastAGI Reference: http://phpagi.sourceforge.net/phpagi22/api-docs/phpAGI/_phpagi-fastagi.php.html
To respond to your specific capacity requirements: you shouldn't have an issue. Stay away from server virtualization when it comes to the Asterisk server, and use moderately-capable server hardware, and you should be fine.
In the interest of not hammering the Asterisk instance with read-type fastagi commands, such as polling for extension statuses, you may want to consider a caching mechanism on the PHP side that polls on a regular interval and stores this information for any number of individual client/browser data requests.