I personally believe in quality, the higher the better, over quantity of posts per day. Although some of the most successful Steemians post multiple times a day, the key thing is to post high quality content that adds value and is useful for everyone.
Since I aim for producing quality content in each blog post, I can't post more than once in a day. I also have a day job so I can only spend at most 3-4 hours preparing a single post. This makes me have even greater respect for people like craig-grant and trevonjb who can post multiple times a day and remain successful on Steemit!