Since my posting, I have been in contact with one of the developers regarding this.
The answer I got was that it is really a limit of seven and the changes necessary was not trivial.
We do not think eighteen will ever be needed. The management of hardware (not to mention the limitations of USB ports) would put a stop to that. If more than 10 is ever involved, it will be possible to naturally split them up into groups dealt separately.
It would be feasible to use more than one computer, but not one per tracker.