Apache or FCGI?
In python world you have few Django processes spawned by WSGI/ASGI app container, and those processes spawn python threads to handle requests. Because python uses GIL, no more than single thread is executed at single time per process. At specific times GIL stops running bytecode for one thread and moves to next one (eg when you do IO). This means other threads keep running, but in theory you can run out of threads.
In async you use tasks instead of threads for http requests, so single process can handle multiple requests at same time, only spawning threads at limited number of situations.
