How to count only the number of empty rows ?

Hi Gurus

I am facing a problem related to displaying the count of specific object([Legacy Postal Code] and [Cleansed Postal code] in my case)which are having empty records.I have used IsNull() but it fails for those set of records which are having Numeric values and not displaying the exact count due to this.The IsNumber() function is not working as well.Could anyone help in providing the solution for this?Attached below the screenshots for reference:

case 1:

case2

case3

The legacy_count formula: