I would believe a Fireman could supervise two boilers if adjacent, or more, if in the same space. However, we have two tasks, one boiler water level, and firing rate, which might require controlling combustion air supply. This might require two per boiler.
As to coal heavers, I would check to see how many were required on locomotives of the time. That's a tough task, I would think at least two per boiler.
The bottom line is that we need to know the required tasks.
If you have the crew list, I'd divide by two, port & starboard, then see what you have in the number of ratings.
Interesting subject, I've been Chief Engineer on two steam driven, oil fired ships.