The plugin get's the number of results per page option from the system option from the Settings -> Reading panel, which is currently set to 10: https://i.imgur.com/ptW44cU.png
If you change that option to 20, it will display 20 results per page.
Unfortunately I can't add support for various theme options for results numbers, because all of them are different, and it would take endless time to check all of them.
I think I might add an additional option to the ajax search pro limits panel for the next release, to control the posts-per-page option separately. That would solve this problem more easily.Best,
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