By that logic, a basic Core2Duo running at smiler speeds would adequately support my current modpack, yet it does not. Multicore usage increases as demand on the server increases proportionally. This is far less efficient then dedicating individual tasks to individual cores however, still better then forcing a single core to run every server calculation.
If you don't believe me (which appears to be the case) you can test the Minecraft single core myth yourself. Grab VMWare or Virtual Box, set it to a single core and, install your OS of choice. For my tests I installed a 64bit copy of Windows XP I had laying around. I was unable to produce a stable tick rate under a single core. Alternatively, tinker with the processor affinity.
Here are some basic tests having done just that: https://i.imgur.com/A11IhZI.png It does not paint a pretty picture. I could barely type out the TPS command so generating chunks is out of the question.
Two cores: https://i.imgur.com/NvsFtvc.png Only a marginal increase in tickrate. I'm not generating chunks under this load either.
With all four cores activated: https://i.imgur.com/CPzKpJ5.png I've circled a few things. Red circles indicate idle activity. Blue circles indicate chunk generation. The increase is near symmetrical across every core, even a few of the extra threads are being used. A single thread yes, but scaled across multiple cores.
If you're hosting a server, and for what every mysterious reason you wish to leave yourself with less headroom don't let me stop you. In layman's terms, you have two full glasses of liquid, and four empty glasses of equal size to the first two glasses. If you pour everything into a single glass some of the liquid is going to spill over. If you pour everything into two glasses they'll both fill up without spilling however, there's no more room for anymore liquid. What do you propose is going to happen when I come along with two more glasses of liquid? Now, if distributed across the four empty glasses you're left with plenty of room for more liquid. It's a rather juvenile explanation, but it works.
I don't have a machine with sixteen cores or multiple physical processors so I can't test against that platform. So long as you're using a modern OS with an updated version of java it should still scale across the two chips.
. To be frank, I'm not sure where my mind was in my initial post. Using a bit calculator leaves me at 102.4mbps not 128. This is based off of the assumed 256kb/s (Kilobytes not bits). Hopefully, now that I've had some sit down time to re-read my posts, this makes more sense to you. You're going to need a datacenter plan that allows for 1Gbps if you expect to see fifty concurrent users on simultaneously. Sorry for the initial confusion, I was juggling several activities along side writing that post.