I have a scenario to do load test.

Ex : Each user has to request to 10 urls in the application.This scenario has to be done for 500 or more users . 

But when I am trying with 20 users it's giving errors ("500-Internal server error")up to 15 users it's serving properly . where it is went wrong from server side or coding side and also I want to know what would be the server configuration for serving more than 500 requests