In it's simplest form, put this in A1 and drag to the right.
=COLUMN(A1)
This will convert column A and so on to a number.
Even though your spreadsheet has letters running horizontally and numbers vertically, the spreadsheet is actually a grid.
To illustrate this on your spreadsheet...
File >> Options >> Excel Options >> Formulas >> Working with formulas>> R1C1 reference style
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