Different languages give different level of control over that. There are languages with one main thread pool and perhaps some specialized ones that users don't have control over. Go would be an example of this.
It is also possible for languages to make user creatable thread pools - possibly even with affinity to cores, allowing fibers to run only on a single thread. Crystal is coming along that path. So far it seems to be coming around fairly nicely but I havn't had to battle the GC in anger yet.
