As per your question its depends on OS, RAM and Processor. You can oversell RAM if you are using OpenVZ. Do it within reasonable limits, otherwise your clients may not be really happy and can be bad for the business in long term. In Hyper-V you can not oversell RAM.
With Hyper-V installed on Windows Server gives you max limit of VM as per RAM means If you have 8 GB RAM on Server that Time you are able to create Max. 6 GB RAM VPS. Like 1 GB RAM Each VPS then 6 VPS allowed.
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