I think 3 is a good number (although I'm fine with 4). I'm sure I have posted 4 or 5 a few times earlier on when I wanted to post a mini story, and before I knew better. I have enjoyed posting a single picture, and then a followup a day or two later. I think for me that is the way to go...

Better to start a story and leave people wanting more than to post too much, and leave people wishing for less.