My theories on train length started with some Iain Rice articles and some practical observations.
Iain Rice stated that for a shelf layout, longest practical train length is 1/4 to 1/3 the length of the shelf.
I don't like to see a train in 2 towns at once. That means the distance between towns should be at least 1.5 train lengths.
Passing sidings need to be at least a train length long, or operations get bogged down. The same is true for yard approach/departure tracks and staging yard tracks. A yard is more efficient if the drill track (yard lead) and at least one yard track are as long as a full cut of cars from the arrival/departure track.
On an around the walls layout, my preference is to limit train length to the shortest wall minus twice the curve radius in the corners.
On an island layout with oval, my preference is to limit train length to the length of the long side minus twice the radius. This is also the longest practical passing siding located along one side of the layout.
Measure your hopper cars (in inches) over the middle of the couplers and divide into the longest practical train length minus an appropriate amount for engine and caboose. That gives you the longest practical train of hoppers. For most layouts, 3 times that number of hoppers would be a practical total on the layout. One train's worth in a moving train, another train's worth in the yard, and a third train's worth at the mine. This assumes your mine produces a train's worth of coal at least once a day, and you have track at the mine to handle it.
For instance, I'll assume longest your longest practical train length is 7ft or 84". Double headed Geeps are 16" long, and a caboose adds 4" (in HO). That leaves 68" worth of hoppers, or 13 hoppers if they are 32ft versions (5"). Leave off an engine and you might be able to use 2 more hoppers. For decent realism, your mine should be set up to use cuts of 3 hoppers each, so a 12 car hopper train works well. 12 cars being loaded at the mine, 12 cars in the train, and 12 empties in the yard waiting to be taken to the mine gives a total of 36 hoppers. If you have a staging yard, another 12 might be appropriate.
As you can see, your operational scenarios, available track, and train lengths drive the number. Very few model mines are set up to handle 4 cuts of 3 cars each, so this will probably be the limiting factor if you intend to run unit trains.
my thoughts, your choices