I'm sorry but Staff have introduced "infinite scroll" to some themes like MistyLook and will be implementing it on all themes that have a suitable structure for infinite scroll. This is a site wide change.
If you have a theme with footer widgets then you can use a workaround to regain control over how many posts are displayed on the front page of your blog. All you need to do is install an empty text widget into any footer area for widgets. When you do that there will be an option displayed on this page > Settings > Reading to disable infinite scroll. Once you have done that your number of posts set on that same Reading page will prevail.
If you do not have a theme with footer widgets like MistyLook then there is no way regain control over how many posts display on the main page of the blog ie. the running page for posts. Your option is to switch themes to one that does have footer widgets and use the workaround.