Kentico by default returns 1000 items for smart search. You can use CMSSearchMaxResultsNumber webconfig key to change it. If a search produces more matching results, the system ignores the items that do not fit into the limit.
I`d say limit it to something meaningful to avoid processing of large numbers of search results with low relevance. You probably do not need more than a couple hundred. Set NumberOfResults to 200 and both sets should be the same.