like @rrd said, we can not simply give an answer to this as there are a whole lot of variables.
- queries being made
- calculations (or math equations) being made (if any)
- how caching is being used (does it keep writing to and reading from the disk for every query, or does it use a cache?)
- tuning (how nicely are your configs tuned for your specific app)
A simple blog site would, for example, hit your servers in a different way than a search engine does, and a RESTful API build in CakePHP would hit your servers in a different way than a social media site would.
I think your servers would generally be bottlenecked more by RAM and disks than by just the CPU.
You can always try a tool like
Vegeta to stress test your setup while you monitor it.