This study is (sadly) not available for free on the Internet, but it is still worth highlighting here. Steven Julious provides some justification for the use of twelve patients per group in a pilot study. This is a useful starting point for discussion, and it may serve as a useful lower bound. I would suggest that you consider the size of the larger trial that you are piloting. For a larger study that might require thousands or tens of thousands of patients, a pilot study of 12 patients per group is woefully inadequate.
Sample size of 12 per group rule of thumb for a pilot study. Steven A. Julious. Pharmaceut. Statist. 2005; 4: 287–291. Abstract available at http://onlinelibrary.wiley.com/doi/10.1002/ps