range selection for autofill

hey, I have some noobiesh question, it's probably been answered before in some way but I have been searching it but haven't been able to find it.

I want to autofill some range with some formula and I'm using these lines.

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only I would like to be able to adjust the range depending on the size of the dataset. so I would to be able to set a number in a cell i.e. A1=20 so that the code changes to

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try this one

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