Basing the rule on a given number of units is likely to lead to disputes regarding which units 'count'. Ex, in the scenario below, where A/a and B/b are forts and armies of the respective players, and x is an army of player B that was defeated the previous turn and cannot move, does army x count as as being placed to influence the retreat of a? Does the fort B count?
Code:
--a--
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bBxbb
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--A--
Instead I would base the judgement on whether the units in question have some legitimate purpose for being where they are, regardless of number.