Option 1: freqs of top docs

    IndexSearcher searcher = ...;
    Query query = ...;
    Term term = ...; // term that we want to count freqs of
    TopDocs topDocs = searcher.search(query, 10);
    for (ScoreDoc scoreDoc : topDocs.scoreDocs) {
      int leafIndex = ReaderUtil.subIndex(scoreDoc.doc, searcher.getIndexReader().leaves());
      LeafReaderContext leaf = searcher.getIndexReader().leaves().get(leafIndex);
      PostingsEnum postings = leaf.reader().postings(term);
      int numOccurrences;
      if (postings.advance(scoreDoc.doc - leaf.docBase) == scoreDoc.doc - leaf.docBase) {
        numOccurrences = postings.freq();
      } else {
        numOccurrences = 0;
      }
    }

Option 2: freqs from a collector
    IndexSearcher searcher = ...;
    Query query = ...;
    searcher.search(query, new Collector() {

      @Override
      public boolean needsScores() {
        return true;
      }

      @Override
      public LeafCollector getLeafCollector(LeafReaderContext context) throws IOException {
        return new LeafCollector() {

          Scorer scorer;

          @Override
          public void setScorer(Scorer scorer) throws IOException {
            this.scorer = scorer;
          }

          @Override
          public void collect(int doc) throws IOException {
            int numOccurrencess = scorer.freq();
          }
        };
      }
    });