public long getRemainingBucketCount() {
        return progressToken.getTotalBucketCount() - progressToken.getFinishedBucketCount();
    }
@return The total number of iterable buckets that remain to be processed

Note: currently includes all non-finished (i.e. active and pending
buckets) as well
documentapi/src/main/java/com/yahoo/documentapi/VisitorIterator.java