How many external reviewers is typical for a tenure case? If someone is on a tenure track in a science or engineering department at U.S. institution (Tier 1, if that makes the question more focused), and it is time to review his case to be tenured and promoted to associate professor, what is a typical range for the number of the external reviewers for his file? Does the number vary if the case is for professorship promotion?
The actual number varies by institution and, frankly, does not matter that much.  The real questions are:


Are you known in a community?  Do people in your academic community generally know of you and, more importantly, your work.
Does the university want to keep you?  It is always possible to find people who will write either effusively positive or cynically negative reviews of anyone and everyone.  Such reviewers are generally known to people who seek their reviews.


The first question is loosely tied to the visibility of your research, and the second question is loosely tied to your value within the university (often including teaching, grant funding, collegiality, and the like).