how to use the workload model(choose between closed/open)


I am using gatling to test the performance of my application. for the first time I started to use the load model: simultaneous
injection of the users(atOnceUser), I start with 50 and then I increase to, 100,200,300 , 500 virtual users

i want to know which is the most appropriate user injection model to be able to start with 50 simultaneous users then increase to 100 then add 200other users until reaching the total of users 500 and how specify all of these numbers in the same setup.