كَمْ رَجُلًا لَقِيتَ؟ — How many men did you meet?
رَجُلًا is singular accusative; this is a question.
كَمْ رِجَالٍ لَقِيتُهُمْ! — How many men I met!
رِجَالٍ is plural genitive; this expresses abundance.
First establish the meaning and context, then identify the tamyīz case. Kam itself is indeclinable in both.
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Al-Kināyāt — Kam, its Tamyīz and Syntactic Positions
كَمْ رَجُلًا لَقِيتَ؟ — How many men did you meet?
رَجُلًا is singular accusative; this is a question.
كَمْ رِجَالٍ لَقِيتُهُمْ! — How many men I met!
رِجَالٍ is plural genitive; this expresses abundance.
First establish the meaning and context, then identify the tamyīz case. Kam itself is indeclinable in both.