Go does both preemptive and cooperative multitasking, so there is no relation to the number of goroutines "running" and the number of threads (or for that matter, cpus or cores).
How this works in Go ? Thoroughly simplified it happens because at every flow control statement in Go, including function calls, what really happens is that, first, Go asks it's runtime to switch goroutines, and only then goes into the if, or subroutine, or ...
