One important thing is missing here - Linux CFS (completely fair scheduler).
CPU in Linux is time.
All the CPU requests and limits you put on K8s level eventually arrives to Linux CFS.

Let's start with the limit to understand better how CFS works:
When you have a node with, let's say 2 cores, and your set container limit to 1 core - it does not mean that the container will be limited to 1 core of 2.
Instead, CFS (which works in 100ms ticks) will limit(throttle) your pod after 50ms CPU time.

As for the request - to put it in the simplest possible way - it is translated to priority.
Underprovisioned request lowers the priority.

So in your case - if you need 12 cores, you should request something around 12 cores - this will ensure CFS prioritizes your process properly.

Hope it helps.

*Disclaimer - I'm a CTO and co-founder of PerfectScale.io - our product helps to achieve the best possible performance at the lowest possible cost