If you mean limiting the live search results, you can change that under the General Options -> Limits panel: https://i.imgur.com/b77W2bk.png
I believe you are looking to chage the first option there from 10 to 15.
The option under the Logic & Behaviour panel is to adjust the posts per page limit if there is a redirection set on certain actions. It does not affect the live results.Best,
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