How to set the appropriate thread number for my server side thread pool?

I just want to ask a rookie question: how to set the appropriate thread number for my server side thread pool?

• Are there any general rules or formulas that I can follow?
• What problems should I consider? For example, the number of network requests per second, the number of processor cores, the frequency of CPU and memory usage in my application, the hardware that I use on my server, etc.