Docker Container capacity planning
The same as any Linux load, consider no more processes in wait state than the number of CPU.
The load on 1, 5 and 15 mins given by uptime should ideally be 1 less than number of cores. 

Containers are roughly isolated processes, leaving a core for orchestration avoid congestion. 

That doesn't mean only 7 containers on a 8 cores machine, it's a matter of simultaneous load, so the best way is to measure it and adapt consequently.