In a combat involving multiple units, you will first need to calculate the total combat resolution score for all units involved on both sides to determine the overall winner of that round of combat.
After this, the Break test modifier for each unit on the losing side is to be calculated separately and compared to the combined combat resolution score of only the enemy units they are in base contact with, not any other friendly units involved in the combat. This means that you need to keep a tally on the number of Wounds caused by each unit so you don't lose track of their individual combat resolution score. This is most easily done by keeping the slain enemy models of each unit behind the unit that inflicted the casualties until the final combat result is calculated.
Units on the losing side whose combat resolution score is equal to or higher than the combined resolution score of all the enemy units they are in base contact with do not need to take any break test, but instead counts as fighting to a draw.
In addition, no unit on the losing side will suffer a negative modifier greater than the overall loss of their side, even if that unit itself might have a much lower combat resolution score than the units they are in base contact with.
Note that if a unit is Disrupted from any source, it may not count its rank bonus against any unit it may be in base contact with.
Example (see diagrams below): A unit of Empire Halberdiers is fighting unit a unit of Orcs to its front and a unit of Goblins to its flank.
The Halberdiers inflict 2 Wounds against the Orcs and 4 Wounds against the Goblins, have 3 ranks, Standard and Outnumber, for a total combat resolution score of 11.
The Orcs inflict 4 Wounds against The Halberdiers, have 2 ranks and Standard for a total combat resolution score of 7.
The Goblins inflict 1 Wound against the Halberdiers and have Flank bonus for a total combat resolution score of 2.
The Orcs and Goblins together has a total combat resolution score of 9, meaning the Halberdiers win the overall combat by 2.
The Orcs have a total combat resolution score of 7 against the Halberdiers. The Halberdiers have a total combat resolution score of 7 against the Orcs. The Halberdiers and Orcs count as fighting to a draw, and thus the Orcs do not need to take a Break test this turn.
The Goblins have a total combat resolution score of 2 against the Halberdiers. The Halberdiers have a total combat resolution score of 9 against the Goblins. Normally the Goblins would have lost the combat by 7. However, as the overall combat was lost by just 2, the Goblins must take a Break test suffering only a -2 Leadership Modifier.