Skaven are not brave by nature, but do take courage from being in large packs of their own kind.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
Strength in NumbersURL Copied!
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Units with this special rule add +1 to its Leadership if it has Unit Strength 10 or more, +2 Leadership if it has Unit Strength 15 or more, and +3 Leadership if it has Unit Strength 20 or more. To calculate the Leadership of a unit with the Strength in Numbers rule, first determine the unit's Leadership as normal and then add the Leadership bonus of the unit to this value, up to a maximum Leadership of 10.
Note that a Skaven General confers his basic Leadership and then units modify it with their own Unit Strength. Any bonus from the Unit Strength of the General's unit is not passed on to units within his Inspiring Presence range.