You have Striking Ver 5.1.8? Themeforest has not yet advised us they have cleared it for release. Otherwise we would have updated the panel to show it is available. I have not posted the dedicated release thread for this version for the same reason.
Striking has the WP-PagaNavi plugin integrated into it, although our current version is slightly behind. But if you are trying to run it as an external plugin, on top of the internal build, perhaps this could be the source of the problem?
I do note from wp.org that some people are reporting issues with 2.82 and wp 3.2.2, this may or may not also be an issue. At a later date we will investigate.
Once we have recovered from all the work that went into this release (the weekend) we will look at updating the internal build.
The ability to control number of posts per page is already in wordpress.
It can be found in the Reading settings "Blog pages show at most" attribute.
And when using the blog shortcode to place posts anywhere in the site, one of the attributes is "Count" where you set the number of posts to show per page.