Restrict Top 26 Rows: Depends on how you want to do it.

In a report: Drag the Dimention and measures into report. Click on the Dimention object and use the RANK button on the tool bar. you can specify the the Top 26  or Bottom(Depending on the requirement)  values of a measure. It also provides some calculation like Sum/Percentage

If you want to restrict the Top 26 in the query itself, you might have to create an object in the universe. May have to use the analytical functions based on which you need to Rank the Data.

for ex: use the Designer function : RANK() OVER (PARTITION BY () ORDER BY ())