Today facetStream takes a "bucketSizeLimit" parameter. Here is what the doc says about this parameter - The number of buckets to include. This value is applied to each dimension.
Now let's say we create a facet stream with 3 nested facets. For example "year_i,month_i,day_i" and provide 10 as the bucketSizeLimit.
FacetStream would return 10 results to us for this facet expression while the total number of unqiue values are 1000 (10*10*10 )
The API should have a separate parameter "limit" which limits the number of tuples (say 500) while bucketSizeLimit should be used to specify the size of each bucket in the JSON Facet API.