I assume, that you get your drop-down-list from validity. If yes, put your values pair into a separate range, for example into Sheet2.A1:B5. So that you get the entries
In the validity dialog choose Allow “cell range” and select the range A1:A5. If the drop-down is in Sheet1.A1 for example and you want the number value in Sheet1.A2, then the formula in Sheet1.A2 is
Another way is to use a form control “List box”. Enter the list in the property tab General into item “List entries”, (new line with Ctrl Enter). And in the property tab “Data” bind it to cell A1 for example and set cell content of linked cell to Position. Put the form control directly on the cell A1 to hide the cell value with the drop-down list. Then you can use the position value for the formula
=CHOOSE(A1;0;365;52;12;1) in cell A2.