how to keep exact number of posts per page

I run a photo gallery where I need to have the exact number of posts on each page. However, even though the “Blog pages show at most _ ” is set to 12, the front page shows only 11, the second page 12, the third page 11, the fourth page 6, and so on in some random fashion. I need to have each page equal. I’m using Wp-pageNavi, so I have not used the query_post function to limit the number of posts or if that can be used with pageNavi, I’m not sure how to do so.

Thanks for any help in advance!