If IIS is being used I'd suggest asking experts over here in dedicated forums.
https://forums.iis.net/
--please don't forget to Accept as answer if the reply is helpful--
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I have a website which have 7000 and some time up to 10000+ users online at a time. Please Suggest VPS configuration which can handle 6000 to 10000 visitors at a time
If IIS is being used I'd suggest asking experts over here in dedicated forums.
https://forums.iis.net/
--please don't forget to Accept as answer if the reply is helpful--
In addition to what Dave suggests, you will also need to provide some information about the application. What sort of memory is used by each user? What sort of CPU load does each user put on the system? What sort of disk activity per user? What sort of network activity per user? Without that sort of information, it is pretty difficult to provide any sort of sizing information.