I am not really understand the last part of that? how number of records made by 1?

me too , I didn't understand it

When we create a field number of records with a value of 1, we are creating a new column "number of records" in the data where every value is 1.
Then if we apply the sum of the number of rows columns with respect to the filters, we will get the number of rows.

I agree that it is not clear why "1" represents the Count of records.  When I investigated the "Number of Records" Calculated Field further (Describe Field)  The Default Aggregation is "Sum."  That *then* made sense that this newly created Calculated Field aggregates (or sums).

The value "1" itself is a multiplier applied to the Aggregate.  If you were to input "2," you would receive 2x the number of records and so on.

He wanted to count how many sales that have been sold in each month of the year, So if you go to the Excel sheet and count the total rows in month 1 of the year 2016 will you find it equal to 3257 which is got it from the last calculated field.