Change rclone threads?

What is the default rclone number of threads? How to change the number of threads to one? I want to upload one file per time.

the default is 4

use this flag to change that
https://rclone.org/docs/#transfers-n

--transfers=1

Thanks. I wanted to try a single file upload at a time to box.com, because uploading multiple files with few kilobytes, or bytes, in size to box.com is a pain. But it didn't seem to make a difference.