Add an efficient `Count` method to find the number of items in each index.

Is there a reason why functions like `COUNT(), MIN()`, etc are not provided?
Or is there a way to achieve these using existing functionalities?

Start by reviewing the repository's existing index and query functionality to understand how item counts are currently obtained. The issue asks about Count, MIN, and similar operations but does not define the API scope or acceptance criteria, so clarify whether the goal is an efficient Count method, additional aggregate methods, or guidance on existing functionality.

Written by the indexing model from the issue text.