How to set up fio working with multiple files simultaneously?

[Yijie Zhong <yijiezhong12@xxxxxxxxx>]

I have learned that glibc emulate POSIX AIO via thread pool on Linux
and only one thread would service for one open file
(https://github.com/bminor/glibc/blob/glibc-2.28/sysdeps/pthread/aio_misc.c#L359).

I want to use fio to validate it, so I want to know how to set up fio
working with multiple files simultaneously?

Thanks!