That's in the theme file - " Default number of topics per page on the postlist screen"
That will only apply to anonymous users and users new from that point on - existing users (including you) will need to change their preferences in the display prefs area of my home.
I wouldn't set this number too big. This is what every anonymous user sees - so the more posts that need to be displayed, the more server load there is. So generally you keep this number fairly low (like 10 or so) and registered users can customize everything themselves. This reduces the load on the server a bit and makes things more speedy.
