For me, I don't think it can be a one-size-fits-all type of answer. The important part is rather the pacing, engagement from involved people, and the narrative length if there's already an ending or goal in mind. I'd lean more towards how many sessions based on average length/time availability are there? Are the arc's scenes set to mix with people's day to day RP or are they designed to be a single moment, spread out in a little plot/time bubble?
I kinda view arcs as like the elementary school chunk paragraph. You have some intro, you make a point, back it up with some support, and then conclude, lengthening as needed for each point to make. But this is also because I tend to be OK with knowing how things will end, as the RP is about the journey to get there. For something more open-ended, pacing and finding a conclusion could be a little more tricky.
I kinda view arcs as like the elementary school chunk paragraph. You have some intro, you make a point, back it up with some support, and then conclude, lengthening as needed for each point to make. But this is also because I tend to be OK with knowing how things will end, as the RP is about the journey to get there. For something more open-ended, pacing and finding a conclusion could be a little more tricky.
