The Count option can be used to determine the actual number of values that are present in your selected column.
Most widgets already offer the option to report the actual number of values that the result in the widget is based on. It can be activated with the 'Show N (number of inputs)' switch that is available in your widget settings. However, in order to emphasize this number, you can also report the Count separately by selecting it as the calculation option.
Please note: this calculations is applied to the records that meet the requirements that you have set in your period selection, your filter selection and the default selection for available records for reporting.