Relationship between number of cores and ability to run processes with higher nice values?

When I "flood" my CPU with 8 high priority (nice=-20) OS threads (the number of cores I have), operation becomes "halty" for obvious reasons, but is still usable. Note that when I say "high-priority thread" I mean a thread that was spawned by the same high-priority process. However doing something like 64 threads will completely make my PC unusable. What is the relationship between max priority threads and their distribution between cores? Can we figure out roughly how many threads I need to spawn to completely flood the CPU for a given nice value?