First of all, if you take it more than once, you better make sure that you improve your scores on each subsequent attempt and at least obtain a competitive 19s all across the board during your last attempt. Yes taking it multiple times might show adcoms that you're motivated to do better , but if you don't obtain those competitive scores, then it'll show them that you've reached your potential and your subpar scores will be an indication that you're probably not capable of succeeding in dental school. In my opinion, prepare as much as you can and take it only once.
Actually, if you read through the DAT examinee guide (pg. 4 on the pdf, numbered pg.2) they tell you that there are specific requirements you must meet in order to take it after your third attempt. You have to submit a written request and receive approval. However, it appears to me that they are trying to avoid people who are selling DAT questions, etc...the requirements are basically all forms that show you are, indeed, applying to dental school.
dent2009 is right though. We received a rather lengthy e-mail from our pre-pro advisors on the question of retaking. In sum, they definitely want to see improvement, so seriously contemplate if it's worth retaking, and make sure you are confident you can improve your scores. Really think about if you will have the time to study or if your time would be better spent improving other parts of your application (your GPA, your experience, etc.)