Express this mathematically

Sorry it has been a while. I have three variables., x, y, z. Every X has a different number of Y and every Y has a different number of Z so how do I express the total number of Z?

For example, say you have people, who all have different number of shirts, which all have a different number of shirt buttons - how do you express the total number of buttons mathematically?

It is not just people * shirts * shirt buttons. What is it? Thanks.

Thanks.

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