Author answered that it can handle 500 parallel users with 8 Core+16GB RAM. Seems like that very good script is not optimized to utilize resources.
Is there any way to test ourself on the virtual environment?
You can do testing on your own. Learn Postman Testing tool. You can send 100 or 1000 requests on your sever and see how it responds. You don't have to have 16 gb and 8 core server. Start with whatever you have. You will get the idea.
But when you have 1000 of users using it in parallel, at same time, then you are successful website. At that time you can move to GCP, AWS or Azure and do autoscaling. It expands as more users come online. Its all automatic. You can either use container deployment or auto scaling the instances. Its simple and very optimized. The autoscaling happens only for the time, your customers come online and use system. If they aren't there, you don't pay for the server utilization. So its dynamic billing, you get charged for the time you used the servers.