Fork incoming requests using python flask
This happens only because you're using the development server included. Flask is a web framework, not a webserver.
Concurrent request serving is a task handled by webservers.
You can use a wsgi webserver like uWSGI to serve your flask application.For even more performance you can also delegate static serving to NGINX but for a pure REST server normally is not needed.
With uWSGI you can specify the number of workers (processes) handling in parallel.
Keep in mind that there's no magic in serving a lot of requests.Even if you use more processes or threads you're bound to that number of concurrent request handling.