I'm planning on taking as many as possible. I've already taken NBME 13, UWSA1 and UWSA2, and am planning on taking NBME 15-19 over the next few weeks. I might do a few of the old NBMEs as well. I figure the more NBME practice questions I do, the more I'll become accustomed to their question style.
Plus ever since I found out about this person who took 18 practice MCATs and got a 528 on their MCAT, I've been convinced that practice exams are the key to success. 😛