A saved search that has exhausted its matches looks exactly like one that is working: it just quietly stops adding leads. You find out when the campaign runs dry.
Saved searches now show remaining available leads — how many companies match the query that have not already been added. Visible in the list, and in the API.
It answers the planning question directly: this search has 4,000 left, that one has 12, so widen the second before it stops. Counting uses subqueries rather than joins, so the number appears fast even on searches with large result sets.