MBS-13074 suggests (as an aside) indicating the number of entities in each series when browsing series collection pages. This could also apply elsewhere. When searching for series, it could be useful to know how large the results are. On the sidebar of a series, it would make sense to list the amount of items in it as well (it's not trivial to count them, especially for series with several pages of items).

Right now we can get a count of the number of items in a series as a side-product of the get_entities method (which returns the items and the number of hits). This could possibly be used while discarding the items, if it's not too slow.