The number of provinces a noble controls determines his status or rank and allows all the nobles in that faction to control additional men in battle:
provinces rank control
--------- ---- -------
1-3 lord +4
4-7 knight +5
8-11 baron +6
12-15 count +7
16-19 earl +8
20-22 marquess +9
23+ duke +10
region king +20 (region must have at
least 25 provinces)
In addition, if a noble has control over every province in a region, and the region contains at least 25 provinces, then the noble is given the rank of king.