Currently reading The world atlas of coffee (James Hoffman)
"You would need to grind the beans more finely if you are brewing a single cup, and more coarsely if you want to brew more"
However when reading elsewhere they mention to keep grind size the same when changing batch size and instead change the rate of pour
"In order to double (or half) a recipe batch of coffee, while keeping the same flavor and intensity itís best to keep the ratio, time and grind identical to the original batch.
This is easy for the French Press - simply multiply the coffee and water weight by the appropriate factor. For a manually pouring method, you need to also adjust the rate of pour. That is, for a double batch pour twice as fast, for example, in order to keep the total time the same.
The trickiest case is the autodrip machine, which has a xed rate of pour. In this case, you do have to change the grind, which will also have some effect on the brew time. For a larger batch on a drip machine, make the grind incrementally coarser until you get the desired result. Use a finer grind for a smaller batch."