Does string override work with singular / plural form?

Yes, you'll have to add both cases. The exact string that's passed through the t() function.

Thanks for the info.

In search_api contrib module,

File : search_api/src/Plugin/views/filter/SearchApiFulltext.php

$msg = $this->formatPlural($this->options['min_length'], 'You must include at least one keyword to match in the content, and punctuation is ignored.', 'You must include at least one keyword to match in the content. Keywords must be at least @count characters, and punctuation is ignored.');

I am seeing no change, when I override both strings of $msg.