Combiner Poles is the number of DC input circuits that defines the size of a combiner box (1 pole = 2 conductors, with no ground connection).
This link shows you how to add combiners (See# 5): 4. Electrical Design
Example: your system has 30 strings, and a "Combiner Poles" value of 12. The math will be: 30 / 12 = 2.5, which rounds up to 3 combiner boxes. HelioScope will then divide the strings as evenly as possible across those 3 combiner boxes, so they'll each get 30 / 3 = 10 strings per combiner box.