There are multiple ways of showing sample size for each column in a table using built-in context menus and rules.
Method 1 - Add statistics to your table
- Going to object inspector
> Data > Statistics > Below > Column Sample Size will place Column Sample Size at the base of each column. Note that you can rename Column Sample Size using Anything 
> Table > Modify Text in Selection from the toolbar (see How to Change Statistic Names and Quarterly Date Labels). Note that when the sample size varies within the columns, this result will often be misleading (see NET is not 100% for a description of the general problem).
- Going to object inspector > Data > Statistics > Cells > Column Sample Size will place Column Sample Size in each cell. This is appropriate when you have differing sample sizes within each column.
Method 2 - Use rules to add sample details to your table or visualization
Note - See the Requirements section of each linked article below for more information about applying rules directly to visualizations.
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Data > Rules > Plus (+) > Row/Column Labels > Add Sample Size to Column Labels from the object inspector places the sample size in the column headers.
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Data > Rules > Plus (+) > Caption > Add Column Base Sizes from the object inspector adds sample sizes for each column (other than the NET and SUM columns) to the footer.
Next
How to Use Rules in Displayr
How to Add Sample Size to Labels in a Table or Visualization
How to Show the Sample Size on Column Headers