POPFile must have a minimum of two buckets which, most often, are named spam and allowed or spam and inbox. Note that “unclassified” is not counted as one of the two minimum buckets. “Unclassified” is only used by POPFile when it cannot make a solid determination as to which bucket an email should be assigned.
There is, effectively, no maximum number of buckets. Some users report how many buckets they are using and these reports are summarised on the POPFile Real-Time Statistics page