Answer
Top N is enabled by default for bar charts and controls how much data is fetched from the database, but the option only becomes available once the chart's underlying Data Source is correctly configured — specifically, the columns need appropriate data types set, and a genuinely numerical column needs to be selected for the Y axis, since Top N works by aggregating/limiting on that numerical value. The Lobby Element Designer highlights the columns it considers appropriate for X and Y axis selection in bold when using those dropdowns — if the Y axis column isn't actually numerical (or the data source's column types aren't set correctly), the Top N option won't appear at all, even though it's enabled by default for this chart type.