By default, CREATE INDEX/REINDEX will create as many segments as there are CPUs on the host machine. This can be changed using the target_segment_count index option. 
CREATE INDEX search_idx ON mock_items USING bm25 (id, description, rating) WITH (key_field = 'id', target_segment_count = 32, ...);
This property is attached to the index so that during REINDEX, the same value will be used. It can be changed with ALTER INDEX, like so: 
ALTER INDEX search_idx SET (target_segment_count = 8);
However, a REINDEX is required to rebalance the index to that segment count. For optimal performance, the segment count should equal the number of parallel workers that a query can receive, which is controlled by max_parallel_workers_per_gather. If max_parallel_workers_per_gather is greater than the number of CPUs on the host machine, then increasing the target segment count to match max_parallel_workers_per_gather can improve query performance.
target_segment_count is merely a suggestion.While pg_search will endeavor to ensure the created index will have exactly this many segments, it is possible for it to have less or more. Mostly this depends on the distribution of work across parallel builder processes, memory constraints, and heap size.