The number of works that you get is related to the number of nodes in the destination array. The higher the number of simultaneous workers, the more throughput you can get.
There is a configured total number of workers for ALL of the SyncIQ jobs on your cluster and if I remember right, it's set to 80 by default. It's changeable, but you should open a support case and explain what you're trying to do and get some input on whether the source and destinations will take the load.
The majority of the load is on the destination side.
Lastly,if you want higher throughput, get yourself on a new release (at least 7.1) of SyncIQ, especially if you have large files. Older release would tie up a stream for the entire file whereas the newer releases will chunk up the file and distribute it amongst streams. Older releases of OneFS also weren't able to drive a 10Gbps port anywhere near 10Gbps so you need a lot of needs participate in your transfers to get the aggregate you're looking for.