Due to the complexity of the problem it is basically impossible to have standard calculation approach.
The required resources can depend on a lot of factors that you would need to include into the equation.
The disc sizes are relative irrelevant. The size you have should be more than enough to install Limesurvey and have a decent sized Database.
What is more relevant is the type of disc and the IO I guess. SSDs usually are quicker and that helps. But the main factor is probably the RAM. The more, the better.
But then, what you need will depend on the size of the survey, the length of the survey, the complexity of the surve, etc. While one survey might run quick and without problems, another survey with a different structure might cause problems. So I guess the only way to find out is testing under conditions similar to your real situation. 500 concurrent users is quite a bit. How do you figure that you will have to handle 500 people accessing the survey at the very same second/time? Sometimes you can also avoid peaks with a clever way to recruit / invite people, etc.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
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