The --transfers option on mount controls number of parallel uploads from writeback cache - so only affects uploads, and only when using cache mode writes or full.
See:
I'm storing them on disk in a parallel directory structure as JSON blobs, so in ~/.cache/rclone/vfs you'll find the data and ~/.cache/rclone/vfsMeta you'll find the metadata.
You can use --cache-dir to direct where you want the cache to go. There isn't a separate setting for the metadata - it is very small though.
The writeback cache allows --transfers uploads at once.
I think that is a separate issue (I'm pretty sure there is one) for a --bwlimit and a --txbwlimit and --rxbwlimit flags. …
If you are using writes or full as the cache mode, then you can use --transfers to control the number of simultaneous uploads. Downloads are still unlimited and can't be limited.