In my experience ONTAP will balance the transfers. If you start 10 baselines, all 10 will transfer and share the available bandwidth to your secondary.
I recently finished initializing about 20 volumes (~5TB of data) and I'd run 2-3 transfers per controller just so I wasn't waiting days to see them complete. There's really nothing wrong with starting them all at the same time, but in my case, I'm SnapMirroring them offsite, so the sooner I had a volume initialized, the sooner I could start the SnapMirror transfer over the WAN.