How many is too many threads in a process?

By default the dotnet thread pool has 250 threads per core[1]. In dotnet, I believe all threads are native, not "green" threads manufactured by the thread library.

I read in other places that 400 threads per core is considered a soft upper limit.

Remember each core, like a car, can only have one driver thread at any time. So all other threads must be inactive -- blocked (i.e. context switched out, or preempted)

[1] My friend told me it's just 25 in older versions.