How do you test an application with over 500 users without using any performance tools?

It is quite impossible as bringing in so many resources is not possible and not advisable as well, it is always advisable to use virtual users for such type of testing, but I believe that if we need to do it manually then my approach would be – create a batch of 10/20 users (A feasible number) and put stress on server by gradually increasing the number of user I will manually see the performance by using task manager, and then can find out a rough figure for 500 as if the response time is 15 seconds for 20 users then for 500 users the response time will be 6 – 6.30 minutes

Same thing can be done by putting load of 20 users simultaneously