You can add this key in webconfig to limit search results to any number that you want:
<add key="CMSSearchMaxResultsNumber" value="3000" />
If this key is not placed in web config, default value is 1000 (first 1000 results in lucene will be taken). From the documentation for k11:
The purpose of the limit is to avoid processing of large numbers of search results with low relevance. If you use very large search indexes that regularly produce more than 1000 results (the default limit), you can either increase the value or set up search filters to help narrow down the search.