You'll need to alter the SQL code of your subreport. However, the SQL used is non-portable. (It depends on the database.)
With Oracle, use the RowNum pseudo-column (add and rownum < 10 to your where clause.)
With SQL Server, use the TOP qualifier. Select top 10 FIeld1, Field2, ...
When using PostgreSQL, it offers the LIMIT (and the associated OFFSET) clauses.
For other databases, check your documentation...
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