Depends on how well I like the RPer and how much time they have to write. If both of us have the time to write multiple posts a day? One. That would be the limit of my focus. Two if it’s a long post so I could be writing one post while waiting for the reply from the other. But that’s happened like…once when I had three RPers who were consistent with regular free days. Has never happened since and I’ve been RPing for the better part of a decade.
It has forever spoiled me to the reality of real RPing. My standards are down to the point where I would accept a post once a month if I thought we could keep the story straight in our heads with that kinda wait in between.