This question already has an answer here:
I am currently developing a website application which should be able to handle around 2000+ users simultaneously when fully operational, what would be the server and bandwidth requirements to handle such capability?
The site is a PHP/MySQL ecommerce site, where users place orders for what they want bought (like a shopping list) and then the order would be complied in the backend side and the lists are then processed. If the supermarket is registered with us the order relating to it would be emailed to them and a supermarket interface is supplied for them so that they can be able to login to the system and view their orders. The site pages have an average size of 8.95kb.
It will run on a Linux server, I can provide more info if need be