how to use per_page?

The plugin allows values from 1 to 50, but if you need you can change this in a plugin code (Line 80).

Usually, such limitations have sense.

Hi, yes, I agree that those limitations have sense, I just need to remove them for the time being.
I was able to relax the constraints by applying the change you mentioned, however the numbers are different than expected. I think the api are filtering out the “attachment” or “media” results. (see here: 49 results for generic relevanssi-based search on the website: https://dev.syrian-heritage.org/?s=Mosaic but only 8 results, apparently, for the api https://dev.syrian-heritage.org/wp-json/relevanssi/v1/search?lang=en&per_page=999999&keyword=Mosaic )

By default API returns posts, to query other types you should define which ones to search like (but with your types):

https://demo.erlycoder.com/demo1/wp-json/relevanssi/v1/search?keyword=test&type=post,page