You have 2 home screens, one of which has a calendar on it. If there's no room for another icon on either screen, a new app will cause the launcher (the app you're running when you look at your home screens) to create another screen.
It's confusing - the screen you go to when you press the home button is
a home screen, not
the home screen. You can always long-press a blank spot on the screen, then choose which screen shows when you tap the home button.
But on the TouchWiz launcher (the one that comes with Samsung phones), you have a limited number of apps per screen. If you want more icons on a single screen, install something like
Nova Launcher and you can have as many as 12 rows and 12 columns - 144 icons on one screen. (Of course, you'd have to make the icons so small that you probably won't be able to recognize them, but I normally run with 6 rows and 5 columns - which gives me a lot more space than TouchWiz does.