public int getResultSets() {
        return resultSets;
    }
A Coverage instance contains coverage information for potentially more
than one search. If several queries, e.g. through blending of results
from multiple clusters, produced a result set, this number will show how
many result sets containing coverage information this Coverage instance
contains information about.

@return the number of result sets with coverage information for this instance
container-core/src/main/java/com/yahoo/container/handler/Coverage.java