Composite aggregation or partitioning with the terms aggregation could give you 100% accurate value if you use them in correct way. Refer to their document for their detail implementation. Also, note that Composite aggregation is the new feature in elasticsearch-6 and still in beta phase.
If you are using older version of Elasticsearch(less than 5.0), then you can use "size":0, if your terms count is less than Integer.MAX_VALUE.