Graphene Options >> Slider Options >> “Number of posts to display”
Of course, if you limit it to certain categories, the number of posts in that category is also a limiting factor. For example, if only posts in category “Slider” are to show in the slider, and I have set “Number of posts to display” to 15, then up to 15 of those posts will appear. But if I only have four posts in the category “Slider”, then only those four posts will show up in the slider.
Without a link to your site it is difficult to be more specific. Hope this helps.
Ken