Number of Miniatures
From Ex illis wiki
Indicates the maximum number of miniatures allowed in the unit. In the mustering phase, you can reduce the size of your units up to 50% + 1.
For example, a unit with a Number of miniatures of 8 can field between 5 and 8 miniatures. Note that units with fewer miniatures cost less but are more sensitive to morale penalties caused by casualties since this calculation is always based on the current Number of miniatures (2 casualties out of 8 miniatures is 25% of the unit while 2 casualties out of 5 miniatures is 40%).