Bands, presumably families or groups of families, generally vary in size from 20 to 50 members. Less than twenty members would not support the lifestyle of a band and the system became unwieldy with more than 50 members. 

A biblical example of a band, and its fission into two groups, is that of the patriarchs Abram and Lot (Genesis 13:1). As the original band grew its territory could not support it leading to internal strife (Genesis 13:7). Then the group divided, the process of fission, into two new groups each with a different territory (Genesis 13:8-11). The carrying capacity of a territory is the maximum population it will support for a particular subsistence level. 

In this case, Abram and Lot were pastoralists. Their original band reached the carrying capacity of their territory initiating fission. Note the fission into two groups was amicable. This is typical of fission wherein bands divide due to unwieldy size rather than political infighting. Fission was common in bands and the Abram-Lot example is typical. See also fusion.