There can be unexpected results from adding your code to sheet modules, such as deleting the sheet.......
so it is good practice to write your code in standard modules and call them from sheet modules if required.

Sheet modules are particularly valuable as the home of sheet events, and can trigger code by selecting or editing a cell, along with a host of other goodies

you have a worksheet change event. any change to the worksheet should trigger it, and it should then pass the affected cell as an object variable. Put a break in at the 'if target.count.....' line to see
1 - if it is being triggered (you could have worksheet events turned off)
2 - if it is not calling the subroutine