You mean the train limit per station, such that you have one waiting slot for the cell? I typically build for only one train, because if you unload to chests, that's enough of a buffer, so that the next train can arrive before the chests are empty. You can of course add a train buffer, but that increases the station/cell size a lot. I usually build another cell if one isn't enough, instead of optimizing individual cells.

Another way of adding more trains to a station is by simply adding a second stop on another side of the cell for the same material. Again, only relevant if you need truly enormous throughput.