I experience some troubleshots with the plugin.
Indeed, even if i specify that i want 15 results per page instead of 10, i still have the default value (10).
I changed the number of posts in General Options -> Reading but it still changes nothing.
Can you help me with this matter please ?
I just checked on the back-end, but I am seeing 15 items in the drop-down results list (I tried with an empty phrase): https://i.imgur.com/gdedksM.png
4 pages total, 3 pages with 4 items (12), plus the 4th page with 3 items = 15 results.
For 15 items per page, you need to adjust the results width and the column count under the Theme Options -> Isotopic layout panel: https://i.imgur.com/2WVXRkz.png
The number of items per page is calculated by the width and the rows count, as it depends on the search width as well, where it is placed. So by lowering the result width you will get more columns, and increasing the rows will add new rows as well.