Interview Question for Applications Development Manager at Microsoft:

i answered three, since that was the way we did it where I worked before. The correct answer was two. All that is required is a single physical file for the databases, master, temp, and user created, etc. The second file is for the log. We used three to keep the master, temp and other standard databases somewhat pristine and then used other physical files for all databases created for actual users and projects. The log file is required regardless of what you do.