This is not exact science. There is no formula for this, there are just too many variables, e.g.:
- length of the survey
- type of questions, equations, filters, etc.
- connection speed of the server to the internet (bandwith)
- connection speed of the respondents
- webserver configuration
- etc.
This might also change from LS version to LS version, some might need more resources, others less.
The best you will find are some real life experiences from users here in the forum. Have a search.
But even then: CPUs are not all the same, it also depends how you configured your server, the webserver, PHP, etc.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
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