Thunderstone Search Appliance Manual

Max Docs to Group By

Syntax: empty, or an integer

The maximum number of result documents to process for Group By results. Decreasing this can speed up Group By results and reduce search load, at the cost of estimated Group By result counts, and possibly truncated Group By results. The max (and default) is empty for no limit (i.e. process all result documents).

See here for more details.