J. R. Tracy
Players may each fire a maximum of one artillery Unit in the
Combat Region at the Enemy; artillery fire is always optional.
Says one per Region not Combat round... could argue both ways....
Taken in isolation, it looks ambiguous, but Artillery usage is one step within the combat procedure. As per 9.11.8, if combat continues, you loop back around to Step 1 and repeat the process, including Step 6 (Artillery).